A matched set is obviously worth more - it's more unique, more difficult to find, etc. There's no doubt that's true.
However, finding the market for a matched set - especially with it up to 9 books now - is the hard part. Basically, it's a buyer who is going to jump in with both feet all at once...or try to sell their current collection to cover the cost.
It's not like there's 50 people lined up ready to pay $11K or whatever for a matched set.
If you already have 2-3 books, you might buy an unmatched as opposed to matched, just to avoid duplication. That would make sense.
But nobody's going to pay more for an unmatched set...there'd be no reason to. If you're in the market for a full set, and there were two for sale, one matched, one unmatched, I don't think anyone here would deliberately choose the unmatched set if all other factors were equal. If somebody would - that's great, but it wouldn't make sense to me, that's all.