Empty leg sales can be fraught with aggro:
1/ If the full price payer gets wind of it he can be upset
2/ Schedule changes as discussed above
3/ Cheapens the product
4/ Customers come to only use empty legs and never charter
5/ Marginal income is rarely worth the hassle.
6/ Some empty leg customer I have found to be the more demanding than charterers
Emty leg selling is a compartive new phenomonen but I think the old way of getting the occasional "double bubble" at full rate worked out better income wise than a load of marginal flights. I still have not made a definitive decision as to which way to go but I really do not think a business model simply selling empty legs would work long term, but I may be wrong.