As always, this has been an interesting discussion. I’ll cover a few points:
The reason for suggesting removing Hexchat and Pidgin is more about their relevance, to most users, rather than iso image space. I personally never use Pidgin, nor do I use any of the instant messaging platforms is supports. I used to, years ago, but now G+, Twitter and Telegram have replaced those. I do however use Xchat all the time, but that is mostly because I’m a developer and just about all Open Source development communication happens via IRC.
The seeds and meta-packages in Ubuntu MATE 16.10 have beem completely restructured. Most pre-installed components can now be uninstalled without uninstalling the ubuntu-mate-desktop meta-package too. To accomplish this has required lots of packages in the Ubuntu archive be patched to add MATE support. I’ve beem working towards this for over a year.
I did investigate replacing Firefox with Chromium as the default browser. After speaking to the Ubuntu Security team the decision is simple, stick with Firefox. Firefox is one of the packages maintained by the Ubuntu Security team, while Chromium is not. Chromium is updated on an adhoc, best efforts, basis.
All the default installed applications will (eventually) be listed in the Software Boutique, so you can easily install/remove the components you want to craft your own desktop experience.