It's a no brainer in my opinion to get Shaq for the following reasons:
1) JO is injury prone and Semih Erden is slated to be his backup.
2) We are not going to run the risk of running KG to the ground to play the 4 and 5, and Baby is not a 5 and will be backing up KG.
3) Perk is not coming back until February and won't be in game shape until March. Also, who knows if he will be the old Perk or a shadow of himself and probably won't be able to really contribute until the playoffs if at all next year.
Kwame has proven to be worthless on many teams in 9 seasons. So, Shaq is the best available option. He may not be healthy all year either, but having a true 7 footer and someone who can bang down low is going to be invaluable to this team come playoff time. Who cares if we have 5 bigs. A front line of Shaq/Perk/JO/KG/Baby is solid with good insurance so everyone's minutes are limited. If I recall one of the reasons we lost game 7 was because of poor rounding and lack of length.
Right now we have backups for our starting 5
PG - Nate
SG - Wafer
SF - Daniels
PF - Davis
C - ?? (Erden)
We have Harangody/Bradley/Lafayette/Gaffney
Honestly, Ainge should give up Sheed's contract, Erden, Lafayette, and Gaffney bring in Shaq and sign another wing like Jarvis Hayes who has length at 6'8' to spell Pierce. Not sure what Daniels is going to provide next year at SF. He's more of SG with the shoot.

Roster I'd like to see...
PG - Rondo/Nate/Bradley
SG - Allen/Daniels/Wafer
SF - Pierce/Hayes/Harongody
PF - Garnett/Davis/JO
C - JO/Shaq/Perk