Probably you're correct it doesn't make sense to bond upload channels if they're not going to use it
Bonded upstreams would help heavily congested areas, even if they weren't planning on increasing the caps. They currently use DCC (dynamic channel change requests) to balance unbonded upstreams, and I've noticed a lot of posts from people having problems with disconnections caused by it. It seems silly to use DCC when a D3 modem is capable of bonding.
TW's 5mbps is a lot more than I can get from their competitor in my area. The best AT&T offers me is 3mbps.