Hi,
the two GBIC-Ports on the X4515 are Interfaces like any other in the switch. So You can configure uplinks on them, too.
I can't think of any disadvantage by using these ports. It's more the other way round: if you use ports on Line Cards, the ports will go down if the line card fails. If the Sup itself fails, you usually have other problems than just these two uplinks ;)
Regards,
Sebastian