The value of T3212 sets the duration between two periodic location update. Typical value is 3 or 4 hours.
If too short, then resources are occupied too much with signalling (SDCCH, BTS processor, MSC processor, VLR, etc.)
If too long, then it can happen that MS are not accessible anymore (battery died, for example) but the MSC still believe they can be paged.
The drawbacks of a longer value are rather inconsequential … It looks like it’s better to have a value too long than too short 🙂
Now, be careful than the implicit detach timer “t” set in the MSC/VLR is longer than the T3212. Because if it’s shorter, then it means that “t” expires before T3212: the IMSI is detached by the VLR, but the MS is not aware of that…