Unless your GM hates you or you're in special circumstances, you will be fighting 1 psyker for every dozen regular mooks armed with melee weapons or guns
Well presumably if one of the players wants to play an untouchable, the GM will lovingly craft situations in which that is rather helpful.
Things like this should perhaps be seen as an 'opportunity' by the GM.
(And honestly, lore wise, if the Inquisitor has a Blank in his service, he's probably going to want to send that character somewhere useful.)