I dont know about pure support. More support than bards are, sure. If you consider how many timed boss fights there are in the 4 man dungeons, such as demon wall and the last AK boss, you just need to make sure that a group with 1+ support class can clear those reasonably, even if its just cutting it really close. So as long as you balance the encounters and the dps for support classes well enough that 4 man content is doable with them, they could be a big help for 8 man content.