I'll agree that tracer missile is NEEDED for mercs almost exactly the way it is, but it should probably have its damage value reduced simply for the fact that it gives buffs and armorbreak effect. If they reduced its damage but maintained everything else about it (spammable, debuffs, buffs, relatively low heat cost), then it should function more appropriately as intended.
I can't think of any other skill in-game that can function as beneficially to its user and still dish out comparable values of damage at the same time. If tracer's base damage is reduced, mercs will still tracer-spam initially (as well they're supposed to), but it'll result in the mercs to use it more strategically, rather than being able to spam it to get a potentially guaranteed kill without having to use other abilities.
I'm not saying to cut the damage it deals too insanely, but cutting its damage by 25% wouldn't be too terrible.
This won't result in a drastic change in how a good merc plays right now, but it would definitely force those less skilled mercs that only use tracer missile in pvp to change their tactics.
That's my 2 cents. So, please, lets not gimp the class - but maybe put it in line with its damage potential to be comparable to say a jugg's armorbreaks.