Because then we'll have a clean HD source to work from for restoration projects, the same as some do now from the 2004 DVD set, but this will provide even more res. The colors certainly are off in many cases, but that can be corrected rather well. And then in the scenes that don't have the SE changes, at least we'll have a pretty good looking HD set of the original trilogy.

Plus, the idea of preserving even the 2004 set in HD before further possible changes are made is rather appealing, at least this way Lucas can't mess it up any more before we have it in HD.

And, of course, having the prequels in HD before changes end up getting made to them in the 30th anniversary set is good, too.