There is no reason to have modcp or admincp in robots.txt - first of all this file is public, so you are telling the script kiddies out there where these pages are, no matter what you have named them.
Second and most important, bad bots ignore robots.txt anyway. Yes that's right - robots.txt isn't a law, or a automated restriction, it's a "please don't index this" page with no teeth at all.
Robots.txt is alot like gun laws - only the law abiding obey them or even pay any attention to them.
If google and other good bots index these pages, they generally won't show in search results because they have no content relevance.
If you want the bad bots out of your life installing
THIS GREAT MOD is the way to go.