Outlaw's a nice player, Channing Frye's a nice big off the bench, and Jarrett Jack isn't a bad point guard, but if you can package those guys and get one of the best young point guards in the league, you do it 10 out of 10 times. Outlaw is a good player, but Martell Webster's a small forward, too, and he fits the team's long-term plans much better than Outlaw does anyway.
Long term, you've got Oden and Aldridge in the frontcourt, with Roy at the 2. Pairing Roy and Harris gives the Blazers a very athletic backcourt, with two guys who are very good at getting to the rim, and both are very good distributors. Oden and Aldridge are both good low post scorers with range out to 12 or 15 feet. All you need is a guy to stretch the defense with long range shooting, which happens to be Webster's long suit. Webster's still very young, and the more minutes he gets now, the better he'll be next year. And despite their good play, it's next year (and several years after) that's Portland's focus.
I think Portland makes out really well in this trade. And quite honestly, I'm not sure I'd do it if I were the Mavs. I don't think that Kidd is so much better than Harris at this point that it justifies this deal. Harris has a better PER this year, and defensively, Harris makes more sense for the Mavs than Kidd, since the Western Conference tends to feature quicker point guards than the East.