Maybe something akin to:

While no longer required for the build, cross architecture versions of these tools
have proven useful to have around, and were provided in prior versions of FreeBSD.
There's no convenient way to get this functionality otherwise, and the extra cost
of providing it here is offset by its usefulness

But even that's a bit too long...

If I could describe my build sandboxes setup with even a modicum of brevity I would do a wiki page on it (people keep asking me to). I think the comment you wrote is plenty good enough, thanks.