you can do much more volume and the rake is slightly smaller.

The biggest reason I play multiple sites, when I really prefer Stars software, is that you can play more of the mtt's you have the biggest edge in, in a given bankroll limit - in my case reg speed, non-rebuys.

Turbos are a big part of a lot of grinders schedules for VIP levels (rakeback), easier to autopilot because you have a short stack a lot and there are a lot less hard decisions. The ROI is less (you can check the tables on Sharkscope) so risk of ruin increases, but they can increase your hourly if you can mass-table them.

Whether they are of value to you? - As always, it depends ;) 

What I've been doing this past year is to just register the type of games I enjoy most.  This has lead to much better sessions and much happier life.  Just play the tournaments you want to play and don't worry about what everyone else is doing.