Alright, let me reiterate what Aldaron said (since you guys seem to have ignored / not understood it). You use your STAB water move to hit everything except other water or grass Pokemon. If you expect your opponent to switch to a water Pokemon, you should use Thunderbolt instead of Psychic, because you will be hitting it harder. As for grass Pokemon, the only commonly seen one other than Breloom (who is 2HKOed by a Specs STAB water attack anyways from Starmie) is Celebi, who ALSO resists Psychic. There is NO point is using Psychic, except for ridiculously obscure situations, like last Pokemon Specs Starmie vs. low HP Suicune + mid-health non-scarf Flygon. Rare obscure situations like that are NOT worth putting Psychic on the analysis.
Also, why does combination type coverage matter on a CHOICED Pokemon? You can't switch your move, so you're stuck predicting to begin with anyways. In that case, you're better off either predicting: non-water (use water attack) or water (use Thunderbolt). Ice Beam if you expect Celebi, but I personally prefer the moveset: Hydro Pump / Surf, Thunderbolt, Rapid Spin, Trick on the Specs set. Having a STAB attack for the sake of having STAB is ridiculously stupid if said attack leaves you as free fodder.
Also, I'm taken back by your (RaikouLover's) implication that Pursuit is overrated. Pursuit is arguably one of the best moves in the game and a very commonly used one at that, so scoffing at it when discussing the Specs Starmie set is a big mistake.
Anyways, I don't think there's anything else to discuss here, since we're obviously not going to ever agree on this. You can keep using Psychic on your Specs Starmie, but I will personally not agree to it being on the analysis (and judging from the other posts made by other QC members, neither will the majority of them).