Be VERY careful using Crosscheck. Like you said, the cross check won't find the backups (since they are on tape), so it will delete them from your catalog. At that point you can't use your backups again.

This can also be the case when you back up directly to tape. We backed up to tape directly using TDP/TSM and ran a crosscheck. Then ALL of my backups were gone!

It can also create a mess on tape if you use RMAN to actually delete the backup files. The catalog now thinks they are gone, but the are still out there "floating" on tape. Messy.

So, I would just stay away from crosscheck if you are backing up to tape.

I have a couple scripts to "maintain" the catalog (delete backups over XX days old, etc). If you are interested, please give me your email address.

Good Luck!
Jodie