An interesting idea. Jefferson's contract is pretty obnoxious, but Splitter and Leonard are interesting players.
I think I'd prefer something like Pierce to Houston for assorted expiring young players if we're looking to move him. Jefferson's contract is just too much of a negative value at this point: two more years @ $10 mil for a spot-up shooter?
I think you have to look at it in terms of the fact that Jefferson makes $5 million a year less than Pierce for the next two years. Getting two valuable rebuilding assets in Leonard and Splitter is definitely worth it, in my opinion.
Even if Houston is interested in acquiring Pierce for expiring young players (I'm not convinced), I don't think the deal would have the same value. Houston isn't going to offer any players as valuable as Leonard or Splitter.
Plus, I think from ownership's perspective you need to be able to sell to fans (and to Pierce himself) that you're trading Pierce to a contender. In other words, trading a beloved franchise player like Pierce has to be framed as if it's for the best for both sides, not just "business." The Spurs are a contender, especially with Paul on board. The Rockets aren't.
The cap hit from Jefferson doesn't bother me that much because I view it as money we're going to be forced to spend, anyway. I doubt the Celtics are going to be players for any major free agents this summer or next.