[andrewjl]

It's simpler to justify if the development process is also seen as a knowledge acquisition process. Does the knowledge acquired by re-invention and its effect on subsequent effectiveness and or productivity outweigh the cost of time expended. In many cases, especially in a mature or stable environment, the answer is yes. It's also an attractive method of learning to people who like to think from first principles, as it involves developing a bottom-up understanding of whatever "the wheel" in this case is via practice.

I find that strong opinions either way on this question to be indicative of time preference. My own view is there's a time to do it and there's a time to avoid it. It becomes very valuable when ones navigating an unfamiliar area or is in the midst of a fundamental shift in the landscape. It's best to avoid it if the team or project is an a do-or-die scenario where certain milestones must be met to ensure it remains a viable team or project.

[jariel]

"as a knowledge acquisition process" so yes, if the module is a little bit core and you would rather have it internally, the added risk of time/quality/maintenance may be worth it. Sure.