See http://bugs.winehq.org/show_bug.cgi?id=14980
Essentially, to run Office 2007, you have to set an override for riched20. Since Office installs a new, spiffier version of riched20 in its own private directory, and expects to find it there, isn't it a bug that we don't let it have it?
There's no guarantee that using the private version would help. In some cases it does, in other cases it makes things worse. So defaulting to builtin is preferable, this way at least when it doesn't work we have a chance of fixing it.
If you don't mind hearing from a real user...
I have both Office 2003 (which also installs the private dlls) and Office 2007 on my system. I regularly use Word, occasionally Powerpoint, and I can verify that it solves a number of other bugs besides the one Dan pointed to:
http://bugs.winehq.org/show_bug.cgi?id=15269 http://bugs.winehq.org/show_bug.cgi?id=14041 http://bugs.winehq.org/show_bug.cgi?id=8358
Now, as for the risk of making things worse: with Word 2003, I did originally encounter one minor glitch caused by the override (needed for the thesaurus): it used to trigger a popup error message about a file not found when using Insert=>Special Characters. It did not stop the feature from working (you could simply close the popup), and is no longer happening in recent versions of Wine, but it does raise the question of whether bugs triggered by using the private dll would be considered valid.
Finally, the issue goes beyond riched20--Office 200x installs other private dlls. An override for usp10 is needed to get the equation toolbar working in Word 2007.