jdMorgan - 3:21 pm on Aug 28, 2010 (gmt 0)
It is potentially bad because you've defined those subdomains and you have linked to them, and now you plan to redirect every request for them. This may look like "doorway" or "gateway" pages to the search engines. You may take a hit on your "trust rank" because of this.
Also, it is just not necessary to redirect the subdomains, and adding a bunch of redirects will cause a temporary loss of ranking for those URLs -- maybe a few days, maybe up to nine months -- I cannot predict this, not knowing anything about the rankings or crawl rate of your pages. But redirecting valid URLs should just not be done unless actually required (for example if someone has sued you for trademark infringement and you *must* change your domain name or a subdomain name because a judge has ordered you to do so.)
The code I posted above makes the standard URLs look like one.example.com, and makes those URLs resolve to filespace in subdirectory /one in your filespace. So the URL-structure and the file-structure are different, but that is of no concern to browsers or to search engines.
HTTP clients don't care where you store your files, and you are free to arrange the filesystem in any way that makes site administration easier, as long as you provide functions (such as this mod_rewrite code) to correctly "map" the URL-structure to the file-structure.
It is not my intent to "give you a hard time" here, but I also want to avoid giving you "the right answer to the wrong problem." This happens rather often here, especially when the difference between an external redirect and an internal rewrite, and the difference between URLs and filepaths are not clearly and demonstrably understood.