I haven't had a "current" game in the past few years due to family, but several times I was tempted to run some single evenings, or short series. Difficulty of preparation and time required (consider that I have become rusty with the 3ed rules already) has been the main obstacle, to the point that I had already decided that I would have "switched" to BECMI to make my life easier... then 5e was announced, and since it appears that it will resemble 3e enough to be familiar to me BUT easier to prepare and run, I will very likely "switch" to it when it's published. In the meantime, I'll run with the playtest rules or with BECMI. But again, since I'm going to run standalone adventures, even if I can use some older homebrew for the setting I'm not strictly going to need to "convert" anything, really...