Having both is no different than having more than one guitar, or more than one amp.
Either product can do things the other can't do (or, can't do as well, for a particular user's needs). I used to have several brands of modelers, and some VST Amp Simulators, and would use whichever had my preferred version of a particular type of sound. I still have the VST stuff, but haven't used them for six months or so.