I would always advocate using collocation under the following circumstances.

1. You have a tape library
2. that tape library is big enough to handle your entire backup cycle - its
really painful using a library as a manual rack cache
3. that tape library can handle the number of exchanges per hour that is
required to offload disk pools within the operational window
4. you remember to set maxscratch appropriately
5. the total capacity of any one piece of media is not disproportionately
small compared to the amount of data a node (or filespace - remember there's
also collocation that way) will send and its retention time. (explanation:
clearly with a 366 day retention period and a large %age change rate each
day then a 100Gb node is going to spread across zillions of 20GB 3590s -
oops I mean small capacity tapes making collocation a waste of robot cycles)

I would happily discuss in detail with anyone who thinks collocation uses
more tape media than the full backup cycle and is therefore expensive.

Regards
Alan