In my mind, there are two reasons to get a mech mod:
1) You like the form factor. This was the primary reason I wanted my first mech. It's small, it's pretty, and there's little that can break. Mechs are pretty much the most reliable/fixable pieces of vape gear I own - should keep me going strong well after the FDA has banned everything but the crappy disposables… ;-)
2) You want to run massive coil setups. It's still the only really viable option for running, say, a quad-coil build (and even some of the hotter dual-coils). Mechs will allow more power draw than any regulated mod.
The biggest drawback (apart from safety, which can be mitigated), is the lack of power regulation. Mechs will run hot (maybe even too hot) with a fresh battery, will settle into it's sweet spot at around 3.9-3.7, and then start limping along until it's time to swap out the battery. Regulated mods give you consistency from the first hit until the time the battery needs to be swapped out (actually, a bit before that point, IME, but that's the concept…). You also need to swap batteries less often with a regulated battery due to the power boost capability.
One of the biggest things I don't get around the "OMG! Sub-ohmming is CRAZY!" hysteria is that no one can get particularly good performance on a mech *without* building a sub-ohm coil.
At a battery's most consistent operating voltage (3.7v), a 1 ohm coil is only pulling 13w - hardly "OMG! Cloud chasing!" power (and honestly, it would be a super-mild, cool vape at that). If you really want a nice warm vape with good flavor and decent vapor, you kinda *have* to build a sub-ohm coil. Or swap batteries every half-hour...
Honestly, one of my RDAs is currently set up with a .7 dual-coil, and I have to swap out the battery at 3.6/3.5v, as the vapor at that point is inferior to a Kanger Mini ProTank. I'm not a cloud chaser, but I like a decently warm vape with good flavor, and that just doesn't happen (IME) above 1 ohm on a mech.