AFAIK, there's a few reasons to use IMAP over the gmail app. Keep in mind, I last used the gmail app 2 months ago, so things very well may have changed:
1. The gmail app has a lag in terms of receiving mail whereas IMAP is virtually instantaneous.
2. You can attach files to the native mail app, you cannot do this with the gmail app
3. IME, the gmail app had to be on the homescreen. I would constantly miss new mail because the native notifier on the homescreen does not tie in with the gmail app.
4. I can't inject items from the gmail app into ToDoMatrix (very specific and may not apply to most people, but necessary for me).
That's what I remember off the top of my head. Personally, I would much rather use the gmail app for the labeling capability, and two way sync. But the items above killed it for me, especially issues 2 and 3. If anyone knows that these issues have been addressed, please post! I'd go back to the gmail app in an instant.