I have to add;
If your reason to buy outdated film is that you want that specific film and its characteristics but it has been discontinued, it may not be very wise after all. Even if it is still usable, the properties like color rendition, contrast, grain etc. may have already changed significantly, and it's hard to say before testing if they have and in which way. So it will be a "different" film after all; and then you don't have a reliable supply for more. So, you'd just better choose a film that is still produced and buy it as fresh as you can. Then, if you hear it will get discontinued, you can stockpile for a few years. But, if it's already a few years out of date to begin with, I see no point. Especially in color slide film.
The price is another matter. If you have a good deal and don't need to have exactly the same look as with the fresh film, go for it.