You have almost (but not quite) hit the answer to the "Build/Buy" debate. Open source is a great answer in many occasions but the "only develop what's missing" is the bit that should be "join the project and add what's missing" - that way you contribute back for the savings you have made in not having to develop the other bits and you also remove the problems of "re-merging" your enhancements whenever the core project updates.

Of course open source is not always the solution - but there is more and more quality open source software out there - what is often missing is the ability to contribute