2 reasons come to mind:
1) Special Teams
2) If you IR him, there is no way he plays this year
#1 was discussed, #2 is a better option for Al since he'd have the option to play this year, and potentially for the packers - there's a chance he'd clear waivers with his contract/injury, which means a chance he could be available if a roster spot opens due to injuries (small chance, but one none the less).
I thought his skills were waning last year, but still a hard worker and by all accounts great locker room guy. For sentimental reasons, this is a real downer!