While I can appreciate that learning a new version control system/code hosting site is a barrier to entry, it is a very small one. There are two type of contributors:
1) People that send small one-of patches
These people can simply send a patch via email or bug report. vcs doesn't get into it
2) People that contribute larger changes/contribute regularly
For such a person, if that person is not willing to make the small effort of learning calibre's development methodology, that will be a strong indication that the person is not reasonable and his contribution will be, as John put it, more trouble than it is worth.
For example, a little while ago, I decide to contribute to the mechanize project. In order to do that, I had to learn how to use git and setup a github account, which I did:
https://github.com/kovidgoyal/mechanize
As I said before, being willing to follow an existing project's development guidelines is basic courtesy, and without that, the chances of your contributions being accepted fall dramatically.
Finally, as evidence that using launchpad is perfectly acceptable to the global developer community, see
https://www.ohloh.net/p/calibre/fact...mSizeVeryLarge according to which calibre has one of the largest open source development teams in the world and is in the top 2% of all development teams.