I also think they are pretty versatile and have definitely gotten better in the 9th edition book. I can see the one unit of ten with 4 special weapons being a major elimination threat, good mortal wound output, good firepower and strong melee potential all in one unit - plus there are tons of stratagems to amp them up. I've reassigned my psilencer Purgators as sword-wielding Purifiers who team up with the (newly promoted) Purifier psycannons. My idea is that you deep strike them in in turn two. Between various stratagems you can have them cast at 15", get a -1 to Hit against them, a +1 to Hit on their shooting attacks, and 12 S8 -2 D2 psycannon shots... plus Storm Bolters and Nemesis weapons if they can make a charge. Pair this with a FttF GMDK for increased charge potential, or Draigo for re-rolls, and I find it pretty scary. Should take down a fair ammount of stuff. And "only" costs 4CP to pull off. You could always spend more to cast an extra Smite and be casting on 3d6 to maximise super-Smite and super-Flame potential, but then that's a lot of CP in a single unit.
I do think running multiple units can be done, but granted you're not buffing all of them up and you'd need transports to get them into Purifying Flame range.
Not an auto-take in my opinion, Strikes and Interceptors are probably the "better" choices, but there is definitely a place for the purest of Grey Knights, even in a competitive setting.