Sigh.

What it comes down to is that there are three things that influence the choice
of where to install Trinity, and two are very dependent on the policies
espoused by a given distro:

-Is Trinity being installed through a package manager, or by hand?  If the
 latter, all bets are off and it could literally end up anywhere.

-Does this distro normally install KDE4 to a segregated location (/opt/kde4,
 /usr/kde/4, or whatever), or root it directly in /usr, which results in its
 applications landing in /usr/bin, where they can't easily be filtered from the
 PATH to avoid conflicts?

-What is this distro's package installation policy?  Does it adhere to the FHS
 slavishly, consider it useful but non-binding, or ignore it?  Are things normally
 sent to /usr, to /opt, or somewhere else?  Is there a specific reason to violate
 the standard policy in Trinity's case?

If I'm not mistaken, there are people from multiple distros (not just Arch 
people) now embroiled in this argument, and I don't think it's possible for 
us all to ever agree on a "best" installation location.  Let each packager make
the decision on the basis of what's best for their distro; multiple packagers
working on the same distro need to hash it out between themselves (ideally
without drawing people from other distros into the mess); non-packagers
can install to /rahrahtrinity if they want to and are willing to deal with the
consequences.