I assume you are looking at 460G-2s we have also encountered that issue.
This was not an issue on G1s but after comparing the vim modules between the 2 platforms there is a lot more going on on a G2 vim. Our conclusion was that the chips for stacking are not sitting idle in the G2 like they would have been on a G1.
That being said it would be great if we could stack a single switch without burning the 10gig ports.
We also had a GTAC Case for that to figure out, if there is a way to get the Slot:Port Notation for switches with no native stack ports.
Even on our single switch installations which have native Ports, we stage them with "enable stacking" in the default.xsf. Only to have the mentionend port Notation to be ready for the future when we maybee have to extend ports via a second switch.