Defense is plural. The improvement you have targeted is singular. The two don't mesh.
Let me lay this out for those who don't get it(not directed at you or anyone in particular):
Right now this is your team:
C-Towles/Ausmus(you could go after posada, but they have committed to the kid, you aren't upgrading here, and outside of like 3 catchers it's not worth it)
1B-Lance(set, enough said)
2B-Burke(not good, and with loretta, castillo, iguchi on the market, I expect one of those 3 to be the OD starter)
SS-AE(there is nothing in free agency significantly better except a-rod and that isn't happening, and odds of good shortstops hitting FA is slim anyways)
3B-Wigginton(lowell or a-rod are out there, but they traded for ty with him starting in mind)
LF-Lee(set, enough said)
CF-Pence(set, but don't like his defense in center so moving him to right field)
RF-Scott(slightly above avg corner of offensively, blah defensively)
Pence, Lance, Lee are locks and not needing upgrade. There is nothing in the SS market better than everett that they can afford, and they have decided their catcher plan.
That means you can upgrade at SP(nothing good in this market), RP(linebrink would be great) 2B(as I said one of iguchi, loretta or castillo will be the OD starter), 3B(A-Rod ain't happening, and lowell is going to get huge money, plus wigginton was traded for to start at 3b) and one OF spot(tons of good CF options available in rowand, hunter, jones, crisp, cameron)
Also consider this is your rotation:
Roy and a combo of sampson, backe, wandy, gutierrez albers, patton, paulino, woody. After roy it's all about average at best. Also consider you have a thin farm system and no assets you can spare to upgrade the rotation via trade, plus how many stud SP's will ever see FA, not many. Santana won't see it next year, just like zambrano, etc.
If you want to win, and the astros do, you can't go into 08 with this pitching and that defensive outfield. You will see mediocre pitching results and the offense isn't good enough to overcome it. Now replace scott with hunter, rowand, cameron, jones, and perhaps crisp(he seemed to improve defensively in boston) and you lose nothing offensively if not potentially gaining a more consistent middle of the order guy(other than crisp who would fill the leadoff/number 2 hitter moving pence to 5th) but your outfield defense goes from below average overall to very good in one move.
Add a good defensive 2b like castillo or iguchi and your defense suddenly is in the elite, and can mask your average pitching, turning a blah team into a good one in one fell swoop. Not to mention it frees up scott as an attractive trade chip for the pitching help you need.
You can say it's not worth 10 mil to go from scott to one of the good CF's on the market because scott's OPS and theirs are similar, but that's foolish and not looking at the big picture. Your pitching isn't getting better anytime soon as the astros don't have the next Roy in their system right now, so eventually you will have to fix that OF alignment if you want to consistently win. And scott will go at that time as pence and lee are cornerstones of the franchise.
To get the complete OF they desperately need will cost 10 mil in any market, you might as well spend it now and make scott tradeable when he has good value, instead of waiting and potentially watching his value decline. Plus, you better your odds of winning and making your franchise an attractive free agent destination in the future. If you leave that outfield as is right now, with this staff, another losing season occurs, and free agents don't like playing for bottom dwellers.