I disagree.

1. We've already had many years of multiple-browsers-working-differently. Without IETab in the mix. Some web developers are still not getting the message. IETab is just easing the pain of a long-existing problem. From this point of view, your argument is like saying band-aids perpetuate the problem of paper cuts.

2. IETab is used by only a small fraction of Firefox power-users. Not enough to make a dent in the general attitude of web developers. If a web developer is on the brink of making his site cross-compatible, the existence of IETab is not going to sway his decision.

3. IETab might actually have the opposite effect. Here's my reasoning: Let's say that I have certain IE-only web sites that I need to use as a part of my day to day work. If it weren't for the existence of IETab, I might not have switched permanently to Firefox as my default browser. I might have stuck with IE. But hey, here's a plugin that lets me use Firefox as the default, and band-aids the rare situations where I need IE. Do you know what that does? It increases, by a count of one, the number of users who have switched to Firefox. Web developers often use marketshare numbers to decide which browsers they're going to support. By allowing me to use Firefox 100 percent of the time, I've increased that marketshare number, thus increasing the chance that a web developer is going to "get with the program" someday.