6-year minor league free agents are almost never useful at the ML level, because getting to that point means they've survived the Rule 5 at least twice (maybe three times? can't remember). Maybe there's like, a defensive catcher or a designated pinch-runner that you don't want to lose. If that's the case, DFA someone now who you know you're losing for nothing at the end of the year anyway and add the guys you want to keep to your 40-man.
As a side note, you do get an advance opportunity to negotiate with 6-year guys after rollover, but they usually demand salaries far beyond their actual worth, to the point where it's never worth signing them. So if they're not worth a 40-man spot, you just have to let them go, no matter how nice it is to have that 95/95/95/95 defensive shortstop who can't hit chilling in AAA just in case.
As a further, fully tangential, side note, if you have the luxury of keeping a designated pinch-runner on your 40-man, I recommend it. I won a playoff game once because my HBD Terrance Gore pinch-ran after a walk in a tie game in the ninth inning and promptly stole second, third, and home in the same AB. Turned out to be the deciding game in the series, too.