If it costs less, then no drawbacks as far as I can see. Have friends going down on JetBlue out of BOS and coming back on Song; the schedules worked better and the price was very good, so...
One potential drawback is that the TSA can sometimes be wary of folks with one-way tickets, and might be inclined to search you a bit more thoroughly. Of course, if they should ask about the one-way fare, you might just inform them of your savings and return ticket on a different airline.