There's really no point in adding capped dungeons with how easy leveling is currently and lack of any real mid level content. If leveling was slower and there was a need for mid lvl content/gear then I'd love to see it.
yeah there is a reason to cap it, EXAMPLE: farming toto rak GC seals from equip drops are being spammed i know i do it with my monk. and the equip will be usefull if there was caps on instances. instances arnt always about XP the its goodies too. they can always add to them in the future too while leaving a challenge.