Not sure I'm the grinch on this one.  I thought PEPs on this were 
rejected long ago and no one complained afterwards.  After years of 
watching newsgroup discussions and feature requests, I do not recall a 
single request or seeing a single problem that was better solved with a 
rational package.  On python-help and the tutorial newsgroup, I've 
never seen a question about the existing module in the Tools directory 
or even a question on the topic.

I think rational modules are ubiquitous for the same reason as Sudoku 
solvers -- they are cute, an easy exercise, and fun to write.  There is 
some variation in how each chooses to make compromises so the 
denominators do not get unusably large. Also, there is some variation 
in sophistication of the GCD/LCD algorithm.

I had thought the standards for inclusion in the standard library had 
risen quite a bit (best-in-class category killers and whatnot).  ISTM, 
this is aspiring cruft -- I cannot remember encountering a real 
business or engineering problem that needed rational arithmetic (the 
decimal module seems to meet the rare need for high precision 
arithmetic).

All that being said, maybe the module with serve some sort of 
educational purpose or will serve to show-off the numeric tower 
abstract base classes.