Why a rhino? If you want mobility then put him on a bike(Increase toughness to 5 and uses twin-linked bolters) or equip him with a jump pack. Both offer the mobility that you want and then some as well as being cheaper point-wise.
Basically I want a strong CC squad in my army for counter assault so the librarian will be leading a command squad of 9 and they'll all be trucking around wreaking havoc. I've got lots and lots of weak-armored vehicles, so if they waste shots at the rhino it would actually help me a lot. I can also use a rhino to block heavy weapons teams if I'm not having the squad ride in it or after they disembark, which is cool.
The libby really isn't enough of a threat on his own to take a decent CC squad in melee, even with veil of time. He needs the backup of 9 other marines to really kick some butt.