^ Shaymin is just a random example. Perhaps something like Raikou or Zapdos would be a better example; I have used SubPass Mienshao with Zapdos to great effect, and it definitely manages to grab momentum like nobody's business. Biggest drawback is that it's something of a liability late-game, but it's definitely a good way to get an early lead in a match.
Also, Shaymin does indeed care about Slowbro Fire Blast, 50.4% - 59.8% to the standard offensive set from 0 SpA Slowbro. Pretty sure that's a bit of a wake-up call if you get predicted.
Ambipom... is only decent at best. I mean yeah, it can hit pretty hard, but it's really predictable and not hard to beat at all. Mienshao still hits harder overall, and is a superior "free damage" Poke. Yes, Ambipom is better against specific targets and fares better against Ghosts, but overall it's inferior to Mienshao in almost every concievable way.