It is my understanding that the host ports on the FEXs can only have servers connected to them and not switch uplinks - like, say, from a blade switch. The reason, as I understand it, is that the host ports are hard-coded for PortFast and cannot be changed. So, of course you would never connect a dot1q inter-switch link to a port configured for PortFast. You can, but its certainly not recommended for obvious reasons.
But is that the ONLY reason?