Compared to you guys, Im just a humble newbie, but I'd like to add my thoughts as a fairly new player of 0ad.
For someone like me, it is impossible to tell (during the game) whether someone is using ProGUI. That implies, that the advantage you get by using the mod is small in comparison to the advantages you get through random map generation, timing, fluctuation in player performance etc. According to @Mentula in this Thread, ProGUI averages out to a 24s advantage in booming. That's fairly irrelevant (and likely the standard deviation is larger ).
I feel in no way discouraged from playing 0ad just because there are people using a mod that gives them a slight advantage (although I obviously can't speak for all new players and understand if someone feels differently).
Honestly, I didn't even know @Atrik uses ProGUI before reading forum posts, but it doesn't matter, since I know how good he plays, and balance accordingly. It's actually a lot of fun to play with him, because there is always something happening and the games aren't just booming.
That being said, hosts have every right to refuse to play with any player, for whatever reason. No host has to play with anyone if they don't want to. It'd be different if there were game-sided servers, but as long as I'm hosting, I decide who gets to connect to my pc and who doesn't.
Maybe it would be helpful to add a way for the host to see if players use ProGUI? Then people that really dislike playing with others that use ProGUI, can just tell them and put them spec. And people like me, that don't see the game as competitive can just ignore that information?
Anyway, no matter if you believe ProGUI is a cheat or not (it is ), you should never insult someone for using it (I'm not saying anyone here did insult someone, by the way :3). That obviously also applies to other people, you should never insult someone for how bad they play in your opinion.
After all, we surely won't get any more players for this amazing game by fostering toxic behaviour towards newer players or players that want to try some of the great mods that exist for 0ad.