From your posting I understand that this means that you are physically "bypassing" the firewall? I do not think that it is intended that any traffic passes along this path, so the point of redundancy does not become clear to me.
The only point could be when you have insufficient ports on the 3512's and want to borrow a few from the 3548. This can be done and when you use two different vlans's for inside and outside, no one will notice, but ...
A misconfiguration or a defect could mean that you are bypassing your firewall.
At least you are making it physically possible that this can happen. Ever heard of Murphy? This would never be my choice.
Hope this helps you to defend your case!
Leo