I built a Roubo bench a while back, and I just love it, I don't know how I worked on things so long without a good workbench, but one thing that I put on the bench that I was not 100% sure I'd use or like is the leg vice, as I'd never really used one. I'm happy to report after almost 5 years of use, I don't know what I'd do without a leg vice.
Just today I was fiddling with the fit on the joints for the tighwire chairs and I snapped this picture....
.... with the long contact area of the leg vice you don't need to crank on the vice to hold your work like you do with a normal woodworking vice, for working on the end of this chair stretcher, the leg vice works so very well. Yes I use the regular woodworking vice more often, but I have to say I'd not like to be without either vice.
My point in posting this is that if you are thinking about putting a leg vice on your existing bench, do it, if you are building a new bench, I'd strongly suggest that you incorporate a leg vice into your design.