counters Keldeo, Medicham, Zard-X
What if Keldeo runs Taunt or SubCM ? (taunt is not the most common set, but it's always a possibility)
What if Zard X runs Outrage ? (of course this also applies to Lanturn but thats not the point anyways)
What if Medicham runs Thunder Punch ?
252 Atk Pure Power Mega Medicham Thunder Punch vs. 248 HP / 216+ Def Starmie: 226-266 (69.9 - 82.3%) -- guaranteed 2HKO
Hit Starmie on the switch, and since your defensive set runs almost no Speed EVs, Mega Medicham outspeeds and kills.
As for the Paralysis/Burn point, once again it also applies to Lanturn and Slowbro. Apart from rapid spinning (and who wants a Rapid Spinner who's bulk is mediocre at best AND also gets outsped by most of the threats it counters or checks), defensive Starmie has no advantage over other Bulky Water types. And don't even mention Reflect Type, that thing is as unreliable as Belly Drum linoone.