2
Yes and no.
It will still follow the link fine for now, but the CID has a wide range of purposes in sorting links, and although it will work fine now, it may be a problem in future versions of XOOPS if you change it.
I'm not sure it would make a different to SE's anyways though. The major problem with search engines in the query string "?". Although several search engines allow the string, a few still don't.
Google loves it.
That help?