Not sure if there's an aggro system or not (doesn't seem like it at the moment), but increasing enemies likelihood to attack shield users could help.
The other option is to do something to nerf two-hander defence. Our parties warrior at the moment uses a two-hander, he's the only melee and he's never felt under threat of dying at all really. The shield doesn't do enough to justify the massive loss of damage compared to two-handers.
The only other way I could see to make shields useful is if you play a spell-casting melee (instead of a flat warrior) for that extra bit of defence (and still allowing you to dump points into intel)? But I've yet to try that out.
