Each instance cluster is now focused on a handful of different gear types, so DPS looking for a necklace would want to run instances and raids from cluster X, while Tanks looking for a necklace would want to run cluster Y.
Unless each cluster is based around this concept (i.e. cluster X is a group of instances that requires all classes to focus on DPS and requires minimal healing/tanking, cluster Y is all about surviving a set amount of time) I can see this being a bad idea very quickly. If cluster X still requires a tank and healer, and even support, it's going to get pretty nasty out there. Mercenary tanks and healers selling their services in exchange for doing instances that will yield them nothing good, compared to gold items for their DPS mates. Or, more likely, healers and tanks just not running the stuff, and glff filling up with "5/6 for X, need Mini/RK please please I beg you, don't join the other 43 groups looking for a healer. We have pie! *cries*".