I'm a huge advocate of loading everything I can on my Dillon 650, to include precision rifle cartridges.
The KEY advantage to doing it this way is how easy it is to size all your brass in preparation for actually loading it. I can literally dump 200 cases into the casefeeder and have them all resized in 10-15 minutes.
This is also a huge advantage in preparing "wildcat" type cartrides that require several different bumping/forming steps. For example, I was preparing 6XC cases from 22-250, which required a shoulder bump with a shortened 308 die, a FLS in a 6XC die, and then a neck-up to 6mm. I was able to do 500pcs of these in less than 30 minutes with my 650. That would have been REALLY painful on my RCBS RC.
Once you have a bin of "ready to load" brass, the Dillon doesn't save much time in actually "loading" them, but it does create a simple and streamlined workflow.
All that said, there is every reason in the world to have a single stage press on your bench, and you WILL use it. Secondly, if you're going to get a Dillon, skip the 550 and get a 650 or 1050. Reason being, the 550 won't handle rifle cartridges from the casefeeder, negating a HUGE advantage of having the progressive in the first place.