Is it a good rule of thumb to always write code for the intent of re-using it somewhere down the road? Or, depending on the size of the component you are writing, is it better practice to design it for re-use when it makes sense with regards to time spent on it. What is a good rule of thumb for spending extra time on analysis and design on project components that have "some probability" of being needed later down the road for other things that may or may need this part.

For example, if I have the need for project X to do things A, and B. A definitely needs to be written for re-use because it just makes sense to do so. B is very project specific at the moment, and I can hack it all together in a couple days to finish the project on time and give everyone kudos for being a great team, etc. Or if we say, lets spend a whole friggin' 2 weeks figuring out what project Y/Z might need this thing for and spend a load of extra time on on part B because someday we might need to use it on project Y/Z (where the savings will be realized).

I'd imagine a perfect world situation would be a nicely crafted combination of project specific vs. re-use architected components given the project. However some code shops might feel it would be a great idea to write everything for the intention of using it at some point down the road.