Thanks for the suggestion. I do agree it would be useful for some people to have this, but I'm not sure when we'll have an opportunity to add it. The way our UI works for buying stuff doesn't lend itself to handling getting multiple copies of things.
For DRM free stuff, since everything is "Pay what you want," you could pay for N copies and get a single DRM free download that you could install on multiple machines. There would be no special multi-copy license with this though, so it may not be suitable for what you're trying to do. It would be more of an honor system: pay for the number of distinct copies you intend to you.