I sympathize with those saying they'd rather see the roster spot filled by another position, or even another RB with more potential to play. But another angle to consider is that it's not just the Patriots with 10 extra spots to fill, it's every NFL team. That's 310 players that the Pats might be interested in that are already taken by other teams now. As they start to cut the OTA castoffs (Gonzalez, Scaife, etc), there's an even more limited pool of players to pick from to replace them than in years past.
I'm sure there are d-linemen, safeties, etc that the Pats would like to bring in for competition, but they might all be on other team's 80-90. Morris is a safe option that won't need a lot of time to get up to speed, but also isn't tough to cut if another player becomes available.