Depends on your mech type. If you find that battles usually last more than 8 turns, a resistance drainer is definitely worth using.

Here's an example that I commonly use to explain the effectiveness of drainers to my friends.

A battle starts you and your opponent. Your first two action points are spent activating an explosive drone and an energy shield. Your next action point you find yourself at range 3, so you fire a resistance drainer (doing -9 explosive resistance) and then a heat disolver (great static damage, mid-range heat weapon).

How much "damage" do you think your resistance drainer has done on this turn? Well, the display didn't show any, so you must've done none, right? Wrong! Your drainer has actually done 18 explosive damage. 9 extra damage for your heat disolver, and 9 extra damage from your explosive typed drone.

And this is only on the 4th turn of the match (opponent, you, opponent, you)! Every time an explosive damage dealing weapon or special (drones, hooks), your resistance drainer technically does another 9 damage. 6th turn, it could add up to 45. 8th turn? 72. 10th turn? 99. This is assuming you get off 2 hits from explosive weapons and another hit from your drone.

Each time you hit with an explosive typed item, even if your opponents resistance is still positive, your drainer still helped your explosive items deal an extra 9 damage each. Since the opponents resistance is 9 lower then it would've been. This also isn't negated by a resistance kit, since the drainer still made their resistance 9 lower then it would've been.

Hope this helped