They can be used in a pinch if nothing better is available.
Just make sure to poke some holes in the bottom of the cup for drainage.
Another thing is you should think about transplanting them pretty early because those cups are pretty small and the plants get rootbound if left in there too long.
I used 9oz plastic cups for my seedlings and they turned out just fine.
They got transplanted after about a week or so.
i get my clones from my dispensary in styrofoam cups and i just plant them in soil using the same cup. however, i won't do this next time. styrofoam seems to have too much friction or something and when i transplanted my babies this time a bit of root got snagged between some perlite and the cup.
plastic cups have always worked well for me though.
I use foam cups, but like he said, doesnt take long to get rootbound...plastic beer cups work good too. Use them when nothing else is avaliable, make sure they have holes for drainage. Otherwise power to yea, they work.
Does any of this depend on the size of the cup?? I use 20 oz. styrofoam cups for my first transplant, get good root development, and haven't had any problem transplanting them. VV