Maybe nothing but it is another mechanical thing to break or whack and knock out of adjustment. It adds some drag, noise and weight which may or may not be important to you. Many might feel it is visually unappealing to have a singlespeed bike that looks like a multispeed. The fact is that there are alot of singlespeed conversions that have overcome the problems you mentioned. But ultimately these are your bikes to do with as you wish.

Jim