I am strongly in favour of turrets. Not that I actually had a chance to use them, but the concept.
It allows a mech with full arms, with all the advantages you called, to still have weapons pointing wherever, and then there is obviously the question of "why not both"?
Hands help standing up, and if your arms are not full, provide additional criticals that could be hit.
Further, with a multi-target mech, or strong pilot, a turret would allow a large engagement angle while still keeping the main battery pointed forward.
Finally, there's the advantages it could provide in AS.
Of course, the costs aren't always worth it, but at worst it's a crit and 1.5 tons, the heaviest mechs normally have that.