As for many questions the answer is more complex than yes or no.
It depends on what you aim to do and how you do it and what you mean by "need". If you think you need improved life tap (ILT) to do damage, well any warlock can sustain some damage against enemies without mana with wands/pets, thus Improved Life Tap is not needed for damage.
Anyway. What will end up mattering is how much mana you spend, how often, how often you use Dark Pact (DP), the mana regeneration of your pet, if you need to use pet abilities and your end-goal. If you spend low amounts of mana unfrequently (ex: solo leveling, killing a monster every 30s) with a phased imp (good mana regeneration rate, few need for abilities unless you want it to Firebolt at which point it'll lose a lot more mana) you can pretty much continue killing forever without getting oom. If you spend high mana frequently with a voidwalker (taunt might be needed, mana regeneration is low) you will go oom fast and might get aggro.
Personaly I use Dark Pact with a phased imp when I solo/farm because I know I won't kill the next npc that fast and it allows me to never have to drink with the way I play. I use it in dungeons with phased imp because it helps healers focus on the tanks and not me and it means I'll have to drink less often, besides damage doesn't matter too much. I do not use it in raids or against world-bosses or bosses because the regeneration of my pet isn't enough to sustain using it and casting shadowbolts (SB) and it's less effective than LT, which means I have to use it more which means less SB, less damage, and 50% crit dmg gives me more damage with my crit %.
