I guess "Some files were not collected because they have not been backed up" is generic informational message along with the print of Event ID 7133 and not a problem.
If files are not been collected then you will see something like below
Archived i...
Without dtrace it would be quiet difficult to say. you could set dtrace on fsautility and do the steps to confirm
If you use -t with fsautility, does it give the same results?
In this case i would recommend to create a trigger file for that closed partition(since you are using trigger file) when you perform a backup for the next schedule.
There are various other factors which you should consider:
-Number of Users
-Data in/out from exchange on daily/weekly/monthly basis.
-Do you need VC/VV.
-PST migration if any
If you are only concern of archiving then in my opinion it shouldn't ...