Why nominating before a special COULD be a good thing
I'm going to reproduce -- with some editing -- my comment on GreginTX22's site:
First, let me note that I don't really have a dog on this fight. I can see the arguments for having a special election -- particularly the importance of having voters decide -- rather than having the party nominate a candidate.
As I've suggested on my blog, a special election is more likely to be won by Lampson. Although I saw that Lampson says he may not run in a special, I think he would be quite foolish to do so. Generally, Lampson has a much better chance of winning the special than the general, because low turnout elections such as special elections are significantly less predictable. Plus, if Lampson wins the special, then he increases his chances of winning the general. Though there are some reasons why he might not want to run in the special, I think those are outweighed by reasons to run in the special.
Also, a special election *appears* to be more likely won by a Harris County resident. I think that it makes sense that Fort Bend wants to keep a hometown Congressman rather than have another Congressman from Harris (note: I was born and live in Harris County).
There's also the idea that the party wants to be able to pick its own nominee. In a special, the party is not picking its own nominee. Both Republican, Democrats and independent voters from an entire district are deciding the de facto nominee. In fact, there is no Republican nominee in a special. Because of this, it means that a candidate with a large, solid base could be the de facto Republican nominee, despite the fact that most Republicans wouldn't vote for that person (eg, in a Lampson v. 7 Republicans special election, one Republican could be in a runoff with Lampson by winning only 20% of the vote.)
Further -- and I'm not saying that this is true**, but that it's valid for them to believe so -- I think folks in the party believe that they can do the best job of picking a solid Republican who has no chance of being beaten by Nick Lampson in November.
So, I think there are some arguments on the other side that aren't merely selfish back-room motives.
** In fact, you could argue that a special election is the best way to ensure that the party doesn't get someone with skeletons in the closet that could be exposed by Lampson.