To make it quick and dirty - I'm a newb programmer who's looking hard at Pyglet, it looks like a really clean and friendly module to use, unlike something like PyGame which is, even by looking with my own inexperienced eyes, a beast.
However.  PyGame is constantly being used, updated, reused by lots of people and seems to have quite a following.  Pyglet hasn't been updated since January 2010.  Most works of art are never finished, only abandoned - but two years and it's still on v 1.1.4 seems troubling.
So while I might be specifically asking about Pyglet vs. PyGame, I'm also not, because it leads me wonder about other ghostly modules that might be lurking out there, that had promise once upon a time but, for some reason, got dropped or shoved in a corner and aren't really relevant.  Are such abandoned projects not worth the time and brain-space investment?
as the owner of a "dormant" package, my own take is:

a more popular package is going to have better support from the community.  for many people i think this overrides any other consideration.  it's often better to have support for a mediocre package than battle with an awesome package no-one else is using.
and it may well be popular because it's better.  obviously i am biased towards the underdog (see above) here, but it has to be said that if a package never gained many users, perhaps it wasn't a good match to the market.

BUT

if the package is mature and working, and it does what you want, why not use it?  especially when open source means you can fix bugs yourself, forking if necessary.

so, it depends.  all other things being equal - use the popular choice.  if you need support - use the popular choice.  but if a package happens to exactly scratch an itch, and it's open source and out there, i would still consider it.
important qualification: this is for personal projects - for work i have a responsibility to use popular projects so that others can support the code i have written.
python specific rider:  one additional thing to worry about is python 3 support.  if a dormant package is stuck on python 2 i would think twice because long-term there's a real chance it will stop working on default python installs.
Python - Is it worthwhile to invest time in apparently stagnant modules?