Right, if VW is out you are drain hunting, not drain tanking.
Same for Felg.
Happy, believe it or not, you can tank darn well w/ the imp out. Some locks prefer the imp, situationally.
If you really go fast through easy, low yellows and greens, your succy can actually run out of mana faster than you can drain her w/ DP. Oh, the imp use is w/ DP - he is your "mana battery."
The imp seems to rebuild his mana pool faster than any other pet. So, when you have the right gear (sta), and maybe some +dam - and you cut through mobs so fast you don't need a pet doing a little dps - you can actually do well drain tanking w/ the imp - in passive, phase shifted - purely as your mana battery.
If you want a pet to fight w/ you, Succubus or Felh is better.
As to other posters, why do people say they don't need Dark Pact????
I mean, yeah, you don't strictly need DP to grind, but it makes life so much better! It lets you over nuke liberally and end fights even faster - without worry of messing up your "no downtime routine" - because you can just click the mana back as you run up to loot, then run to next fight (i.e. casting nightfall procs, using siphon life every fight, etc.).
DP is really nice! I honestly don't get affliction locks who drain tank and don't get or use DP. It lets you kill much, much faster, and it smoothes out the "rough spots."