RDM doesn't stack with other supports, currently, thus your question is invalid. I'm not going to discuss what ifs for a future that may or may not happen. You are making a large amount of assumptions.

Nothing about our current skill set dictates that the only viable position we can have in a group is as a support mage. That is something you came up with based off your personal vision of the job and what it once was. Welcome to 2012. SE does what it wants with your job and you get to guess what they are thinking because they don't tell you and they don't make a lot of sense. The only thing RDM is actually good at doing, currently (ie In the actual game), is take hits to the face. We could be good at enfeebling if SE had allowed us to be, but they did not, so please don't reference enfeebling as an asset we currently have, because we don't actually have it.

If you want to petition for RDM to become a support mage, then do it. But don't assuming SE is going that way, because we really don't know anymore. Please look at where RDM is actually at right now and not preconceived notions of yesterday or other more dubious sources such as your personal opinions of it's future. After you have taken a good hard look, then realize what you, personally, want for RDM. Because it currently is full of holes and ambiguity that need to be filled.

SE could decide that RDM is at the end of its spell list and that they don't care how poorly the job is functioning right now. It has 19 other jobs that actually work and have their own job identity. It probably can afford to slack on RDM because there are so many jobs that have already taken all of it's territory. Please don't assuming RDM will be good, just because you want it to be, because currently it is not.