10gen also provide a repository. Because projects like MongoDB often outpace the LTS type release in terms of releasing stable versions, there can be reasons to veer off the distro path.
There is a pretty decent description of the differences here:
Should I install MongoDB using Ubuntu's repository or MongoDB's private repository?
As an example of what I mean above, if you stuck to SvenW's policy on 10.04 you would be running 1.6.4 - the first thing you would be told if you ran into an issue would be to upgrade - 1.6.4 is ancient by MongoDB standards. Currently 12.04 gives you 2.0.x which is the current release, but fast forward to 2014 and again it could be looking pretty old (2.2 is due out shortly as of writing this).