Surely the yeast cake would be a good reason to not do this. I get 3-4 cups of slurry in my fermenter, and I honestly wouldn't want to drink that :icon_vomit:
Plus, surely the krausen ring would dry off and go all gunky.
What advantages - save the minimal time it takes to rack from a fermenter to a keg, and the minimal saving on not buying a fermenter - would this provide? Cleaning one less vessel? Not to mention you're using up a perfectly good keg for the week-or-so it's fermenting.
I've mentioned elsewhere that I have doubts over the effect of 'head pressure' on fermentation, but that was for the very minimal pressure you get from an airlock. Unless you go the glad-wrap method for fermenting (nothing wrong with that, but your 'keg' is now just a cylinder with glad-wrap over the top) you'll build up a reasonable pressure in there, even if you burp it regularly.
Summary: You could carb up and serve from the cheaper plastic fermenter, but I wouldn't recommend it.