And if were lucky, maxed out gear will do nothing but drive up some slight bonuses if tactics are involved of course. That depends on the design of the multiplayer and the difficulty though.
Using ME3MP as a comparison, (I expect a 'vaguely similar' principle to be employed in DAMP)
the best ultra-rare weapons were doing about double the DPS of a base weapon (very roughly)
with rare weapons at around half/2/3rds better than base. Cyonan did a load of work on this.
Upgrading the weapons themselves would put another 25% DPS on top from level I to X.
Your skills at playing is capable of increasing more, so learning to play well is going to be just as beneficial as adding 2.5% damage with an upgrade.
It's a bit like levelling your character from level 1 to 20. Levelling 1-5 is much easier than levelling 15-20.
(In fact you expended the same experience points levelling from 1-17 as from 17-20.)
For that reasons, paying micro-transactions are probably of most value (relatively) early on when you are weak.
But those first 100 campaigns are also very precious because that's when you're learning and experiencing the game anew.
I'm sure micro-transactions will be used by some but actually for me it short-cuts enjoyable game play.
It's like being handed a walkthrough or easy quest markers.