my rule of thumb is that if a guy doesn't hit well enough to play the toughest position where he's an average+ fielder, he's not good enough to be an every day player
of course, it's not a huge deal if you surround him with enough studs to pick up the slack. but it's hard to get your hands on a lot of studs without big IFA signings and high draft picks, so if you're playing to win 90+ every year, it really helps to aim for better defense.