To me there is no sense in hoarding them. Their value can vary so much. Plus I pretty much exclusively use the DL Amex cards so I rack up quite a few throughout the year. I use them for anything where the cash price is way too high or for short trips. I had a rather last minute ATL-DTW rt ticket that was going to be $400+ but it ended up only being 19,000 SKymiles in Main Cabin. I've gotten quite a few redemptions like that. I do occasionally use Pay With Miles to make the cost a little bit more manageable for trips for the entire family.
I can't take 'em with me!...so I might as well use 'em.