I think taking grafts from one side depends on a few factors, namely the session size and the quality of the donor.
For example, if you need 1000-1500 from an outstanding donor area, taking it from one side leaves no noticeable change in density. The advantages from a surgical perspective are that you don't have to keep moving the patient around, and also numb down a smaller area of scalp. I'm no doctor but clearly this saves time and reduces risks to a certain extent.
Say someone had a slighly below average donor and you wanted to get 2500 grafts. Cleary it would be better to spread things out than create an abnormally thin side sompared to the others.
I guess you can apply the same idea to FUT. If you just need 500 grafts, you wouldnt go and make a 30cm scar. You just do 10cm on one side, minimising risks of stretching, bleeding etc.