Delurking to comment on this topic as it's one close to my heart (although I don't have much to add and am just repeating things that others have already said better). The first ballet I went to, not all that long ago, was a triple bill. I'm not sure a more traditional first ballet would have captured me that way that did and (although my purse would have been a lot happier) I'd have missed out on a lot of the excellent things I've seen since. I agree with Rowan's comment.
I think mixed bills have great potential to introduce people to ballet but it's tricky to unlock. One of the difficulties has already been mentioned - when mixed bills include new work there is an element of risk in buying a ticket. If you've never seen a ballet and want to give it a go then picking an established, full length, narrative piece is a bit like looking for a kitemark when you're shopping. I think another thing that puts people off mixed bills is the presence (at least most of the time) of abstract or semi-narrative works. If you think you might not 'get' ballet it's probably a bit more within your comfort zone to pick something that at least has a well-known story you can follow along. I'd be interested to know if there is a difference in how different types of triple bills sell (eg those with at least one strong narrative piece or those with no new work) and whether 'introductory' mixed bills with a focus on strong narrative works have ever been used successfully to get people to try a mixed bill who wouldn't normally.