The problem is they'd most likely escape, and then unlike slaves you can't just recapture them, because they are after all good white Christians (I am basing this on the failure to use white labour on early Spanish and Portuguese plantations. The workers escaped, and unlike slaves they couldn't just go and recapture them).
One more problem is that serf, unlike a slave, had certain rights, family and property. He was obliged to work certain number of days for his master and to perform some duties but his master could not just sent him somewhere (serfdom in Russia circa XVIII was not typical for the rest of Europe).