Ondrej, Bernard, and Tim have been sort of arguing in response to Rob Dodier's nice post... The discussion is missing some shades of gray. Here's what we actually do in Sage:
1. Identify needed functionality (e.g., compute characteristic polynomials of matrices, or "symbolic calculus").
2. Find the existing best open source free option that implements 1, if there is any (e.g., say the pari C library in the above example, or "maxima" + a very sophisticated Python interface).
3. Fully integrate 2 into Sage. (Bobby Moretti and I did this for Calculus, in the above example.)
4. Somebody comes along and points out that 2 is not optimal and that they can do better. E.g., they have a new algorithm for computing characteristic polynomials, or think they can implement a drop in replacement for symbolic calculus that is in fact much better.
5. Somebody tries very hard to implement 4. Sometimes they succeed, and we *switch* over to it, and sometimes they fail, and the code never goes into Sage. Both happen and that is fine by me.
This is what is happening with symbolics. In 2005 we identified Maxima as the best open source system given our constraints. Bobby Moretti and I spent about 8 months fully integrating Maxima in as the core of our symbolic calculus functionality in Sage. Bill Furnish "popped up" and claimed he could do something much better, and has been working on this for the last 5 months. If (or when) his code turns out to be clearly better than what is currently built on top of Maxima in Sage, then it will go into Sage. If not, it won't.
The *only* reason I can see for not going from 4 to 5 in the above example is that I, or Bobby Moretti, and Maxima authors, or whatever have big egos and can't stand to see their hard work and code get thrown away. Well I swallow my frickin' pride and just deal with it, and have thrown away massive amounts of code I write. I think that's really the core issue in this whole thread -- some people are really disturbed by code get thrown away... Well deal with it. It's much more important to make decisions that are best for the overall quality of a project and "the community goals" than to stroke your ego by keeping your own code alive forever.