You're underestimating how problematic updating Evolution could be. Two big
problems would arise:
1. Newer versions of Evolution are made to work with newer versions of GNOME.
There could be incompatibilities.
2. Ubuntu isn't updating Evolution, so if Trisquel did, it be on its own
providing security updates for it. That would increase the amount of work the
few Trisquel developers would have to do, or if they just didn't do that
work, it would put the security of Trisquel at risk.
Honestly, I don't think there's any particular reason to favor Evolution for
mail. All I use Evolution for is the calendar (and honestly, I think that
should be split off; I don't get why they're lumped together into one app).
For mail, IceDove works much better, mainly because it's so good at
automatically figuring out how to communicate with mail servers.
