since the first time i have seen the infernals in the wow intro i knew i must have it as a pet for my warlock. at level 50 finally i could summon it but not as a permanent pet i really liked it.
sometimes i really don’t know why things players definitely find fun should be removed from the game (even if temporarily as it may return in the future in some expansion again).
I believe it was made to make the Infernal the dps boost for 30s on the Destruction Warlock to match the fire’y theme of the spec. Demonology Warlocks got the hulking demon thingy (can’t remember the name, sorry) and Affliction spec got the beholder-like looking demon (death gaze was it?).
Altough having a glyph for every pet as a cosmetic would be very nice indeed.