Capped, yes please.

I don't know why they chose not to cap them in the first place. The gear system is already catered to capped content (wear any gear at any level.) So even if you're a R40 you don't have to worry about getting "capped gear."

The R30 dungeons can also be another way to exp aside from leves and open world grinding. Keeping the cap at 30 will award consistent exp throughout all levels, but also can't be abused because of the time restriction that will likely come along with them.

The only reason I can think of for going with uncapped dungeons is to make them easier, period. And that is the LAST thing this game needs. I'm secretly hoping Yoshi-P changes his mind before 1.18 goes live and releases them as capped dungeons.