Just about every third party I have dealt with insist that "Administrative permissions" aren't good enough and want THE Administrator account for some reason, so you're not alone.
During an episode of misconfiguration, we ran Backup Exec services as "Local System" for quite some time without incident- and we had pre/post scripts doing "net stop dhcp" for example, which continued to work.
You should have no issues creating a BackupExec account, running Backup Exec as that user, as long as that user is a domain / local Administrator.
The worst you could do is try it, have it not work, and then change it back to administrator.