The quirks are largely irrelevant--the big deal is the ECM. It's what makes the 3L the best sniper/support of the lot, despite the 4X being an excellent mid-range laser skirmisher and the 2X being a good platform for laser vomit. Almost nobody carries Beagles, especially in PuGs, so you're immune to the usual LRM spam. Likewise, PuGs generally don't focus fire well without red pringles--the only times you're likely to get shot at is when an individual is both observant and carrying a PPC or their own ERLLas. Natch there's also the material advantage of enemies not knowing where you're damaged or being able to pick you out amid cover as easily. Note, also, the "support" part of it. You can give that same coverage to a very good-sized chunk of your team, especially when you're on a map that encourages mid to long range potshotting.
Besides, in that family the Huginn's your boy for missile play. The only reason I can see for fitting missiles to the 3L would be if I was going for a really old-fashioned (like, early-2013 vintage) full support build, with the SSRM 2s, a couple MLas, a TAG, and just hugging friendly Assaults and Heavies while peeling off to kill enemy Lights. The problem with that is that it's only really viable in an organized group--in solo play your teammates generally won't take advantage of your TAG marking or ECM, will repeatedly run into you or shoot you while trying to kill enemy Lights, and you won't be able to do enough damage for a decent score. It's the same old sad story of every F2P vehicle combat arena game: the best way to be a good team player in random matches is to build for damage, play carefully (i.e. as if you were the only human being on your team), bait your teammates, and focus entirely on killing whichever target is least likely to leave you exposed. MWO's better than a lot of similar games in terms of community friendliness and cooperation, but I've still run into the assholes that'll core you from behind trying to shoot past you to steal a kill, the ones who ignore marked enemies circling around to flank, &c. forever.
e: I mean, yeah, you can do the x2 SRM 4 and nothing else ass-pounding thing, but you're still a Raven and thus squishy enough that getting caught at all means you're dead via ammo explosion (or you carry so little ammo that you're useless after a couple minutes if you don't die). That, and the recent change to SRM spread means that you have to be more precise when trying to shotgun other Lights with them. And the Huginn still does this vastly better.