I'd say the only real benefit would be to give you multiple routes to restore the same data, so if for some reason your Exchange backup failed you'd at least have the old mailbox backup to fall back on. Obviously that's a fairly minimal benefit, while on the other hand the amount of space taken up by your exchange backups will be more than twice as much as they would be otherwise, and the time taken to do the backups will go up a lot as well.
So personally I'd say that if you have the tape capacity and available backup time window then go for it, after all there's nothing to lose.