Sending convicts or even lay persons of a non scientific background would make no sense. Until the day when there is a thriving system able to provide food, water and oxygen for large masses of people, not to mention structures to house them; the only people who should be sent to Mars should and will be those who have beneficial scientific, medical or technical knowledge. Preferably individuals versed in multiple disciplines as with a limited number of individuals, millions of miles from "backup" you wouldn't want to be without a doctor or an engineer if someone were to be injured or killed.
Don't get me wrong, sign me up the day true colonization is possible and lay people are needed there. For now that simply wouldn't be the case, what would we do other than use resources we wouldn't be helping to generate.