i honestly think that ojt's should be compensated, it is some sort of exploitation if you ask something from them for free(your belief may vary. for me, it is) .. at least an allowance might be enough.
specially if the ojt's have the basic understanding how the language works. ojt's under mine got like 300/day as an allowance. if the ojts doesn't contribute to the project, then it's a failure on my part why he/she fails. but most of the time, i got more than enough what i need from ojt's.