Not it doesn't. A counter should be able to switch in comfortably and always force out a Pokemon. Key word being always.
-If sleep clause is in effect Parasect loses.
-If Parasect has taken residual damage it loses
Also you can't assume you'll switch in while rain is up. If you switch into SR while the sand is up, you actually lose period.
I'm sorry, but that definition of counter makes absolutely so many things left without counters, I must refute. Regardless of whether or not you believe a pokemon having no counters means it is broken, which is a whole other issue, very few pokes can always switch in on any move of an opposing poke and KO it if it stays in. There are just too many factors in play. Hazard damage plays a big role in that, LO recoil, choiced moves, and so many other factors effect a poke's ability to switch in and KO/force it out. Besides these factors there are at least 3 (or two, depending on how you would group them) factors that above all determine a poke's ability to be countered, prediction (or predictability, to rephrase), crits, and accuracy.
Prediction is very difficult to quantify, yet the overall level of predictability of pokes is frequently qualifiable, and very important to the ability of that poke to do its job well, or even to the point of being broken. Many sets in the past were introduced at a time of the meta being very centralized aaround a few things, and were able to dominate because people did not see them coming. To offer an example, Sub+T-wave Jirachi was incredibly irritating and useful when it was pioneered, yet teams eventually learned of its prowess and its ubiquity, and it became far too countered to be the absolute domination machine it was (much like Scizor of Gen I as well). Certain sets are very predictable, Doryuuzu, who runs the same set so often most teams can easily predict what he will do given a free turn, thus making him less of a monster than he was previously. This is a very large factor in his continued stay in OU, as few teams consistently struggle with him anymore, as they are usually well prepared for his intended rampage.
Following predictability, I see hax and accuracy as two enormous factors in the level of brokenness in a poke. These factors can frequently either contribute or detract from the brokenness of a poke. Things like CM Reuniclus aren't completely unbeatable because they rely on both meh coverage (which isn't really part of this idea, but important to note) and a 70% accuracy move to beat one of its most feared potential counters, CB Tar. Running no SpA, Focus Blast will never OHKO a max HP T-tar (which is the most common CB set), and the likelihood of striking twice with Focus Miss is 49%, far from a reliable win. The list of pokes which Reuniclus needs this amount of luck to win against is longer than one might imagine, and it stands a very convincing sign of Reuniclus's counter-ability. On this same note, things that can only counter another poke with a low-accuracy move are also far from perfect counters, as they require luck to win. On the idea of crits, they affect both counters and the countees, and serve only to remind us all that the concept of a perfect counter can never truly exist in practice.
As for the other factors, such as residual damage and choiced pokes, I will only speak briefly, as they are more clear and easy to understand. Setting up stealth rocks is nearly 99% of the time a good thing for teams to do. Stealth rock inhibits opposing sweepers, helps net KOs, allows for phazing to rack up damage, and only takes a single turn to set up. As for Spikes, they are also incredibly useful, but are more limited in distribution and take arguably too much time to set up if you are not running stall. For these reasons, Stealth Rock is justifiably ubiquitous, and should be considered in most all cases when discussing the brokenness of a poke. When we are referring to a poke like SpecsTios or Scarfers, who's job is to switch in and out and rack up damage, hazards are a very important part of discussion, as it is rarely possible to entirely stop one's opponent from setting up stealth rock at least once. For those kinds of pokes, hazards contribute greatly to their counter-ability, as their "counters" need only to be able to do a certain amount of damage to counter them the majority of the time. Choiced pokes are also part of this category, but should be considered and debated differently for a few reasons. For obvious reasons, forcing out choiced pokes is far easier than forcing out something like LO Starmie (in Gen IV, that is), and thusly hazards are a bigger factor. But having pokes choiced brings another issue into play, the idea of predictability. When something is not locked into a move, it can't always be switched into even on a resist. I would offer Ludicolo as an example for this. Nattorei running no SpD at all can switch into Giga Drains from Ludi all day, but can do little to him and will be 2HKOd at the very least by Focus Blast (which also has an accuracy problem, but that is largely inconsequential in this case). For this reason, switching in Nat only serves to accumulate LO damage which, despite not being necessarily a bad thing, means it in no way can counter Ludi. But for Choiced pokes, prediction is more of an issue, for if your opponent predicts your move correctly and switches out to an appropriate counter, if needed, you have undoubtedly given them 1 free turn to do whatever they wish to harm your team, which is the last thing any player ever wants to do. As for LO damage, that is even more assured than hazards, and should be considered when contemplating the brokenness of a poke, for if a poke can switch in on 3 of the opponents 4 attacks, and then revenge them when they are at 90% health, I truly think that poke should be considered a counter.
For all those tl;dr folk who simply glanced over this, I would hope you at least read this. One can rarely perfectly match up against any other poke, and the concept of a counter should not be derived from that. A counter should be determined from the most likely situation one would find himself in against a particular threat, taking in account all factors of success and of failure. I really do hope you all read this, so that we may further advance the discussion of the definition of a counter and be able to debate on identical terms.
