I think it's very worthwhile to integrate the two. Mantis is an easy to install and configure issue tracker and dotProject is an easy to install and configure project management tool. It seems like a great fit.

As a side note, about 6 months ago, I was in a meeting with my boss and he mentioned that we needed some project management tools. While still in the meeting, I ssh'd into the Redhat box under my desk and had dotProject downloaded and deployed within 15 minutes.

Of course, I had already done it a time or ten, so maybe I'm not an unbiased user...