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Discoloration
Plumbers putty has oils in it that can cause discoloration of plastic models and stain your hands. The oils are not a problem on PVC pipes, which are usually used in plumbing, but can create both unexpected and undesirable results if you try to match paints over the surface of a plastic model to a place you have puttied in.
Odor
Plumbers putty also has a strong, distinctive odor caused, in part, by the same oils. While you might not find the odor offensive, it may not be appropriate to the effect you want to create with your model. Standard modeling putties have no odor after they have fully dried.
Shaping
Putty is used in plastic models to fill gaps and seams. It also can be used to make repairs, because the putty can be easily shaped into just about any form you want, provided you finish it before curing sets in. For complex parts, modelers should use a non-epoxy-based putty because epoxy based putties are hard to sand into fine shapes after they have dried. Plumbers putty has the same problem as modelers' epoxy -- and it is compounded by the oils it has in it.
Epoxy
Epoxy glues, generally, are best used in plastic modeling for larger parts and when a piece of metal must be glued to a piece of plastic. It takes 12 hours for epoxy to completely set. Until it has, unless you have set up a method to physically hold the parts in place, you are likely to find that the hardened parts are not in the same position that you left them in. The best glues for bonding plastic to plastic do not work well for bonding metal to plastic, but epoxies do.