emperorI'm interested to know the thought process of why someone would add up on a class that they actually don't want to play. I'm inclined to think that the scenario where two players can swap to turn the tide of a very one-sided match is if they switched from a class where they had very little impact to one where they had significant impact. But if that's the case, why wasn't that particular player drafted in that main position to begin with? Did the captains just fail to pick that person on that class and explicitly chose to pick them on a weaker class?
Personally, I'm in favor of flexibility and free-form gameplay. But I do feel that because we have an ELO system in place means that players often will bias their decisions with the interest of winning/gaining rating, rather than pug balance. To that effect, by preventing players from playing classes they're not selected as, the onus is on captains to pick the right teams but at the same time both captains can pick players comfortably without having to think, "Did I make a bad choice because I didn't pick that player who added up on demo and not soldier, but is actually an invite soldier?"
I get what you're saying in that you shouldn't add on a class you don't want to play. One example that comes to mind for me is if two non soldier mains are both added on pocket and roamer and get picked on the same team and for whatever reason one of them wants to pocket more than the other one (perhaps its a scout main who likes using hitscan). Another case would be people like rick who frequently add up on every class because they don't really care what they play. Before one of the games started in mumble he asked me why I always pick him on pocket, and someone else who was good at multiple classes asked if he wanted to switch to scout. At the time I didn't consider the thought that it would be 'unfair' to switch the two because both players can play any class and if one of them thought they'd have more fun playing one than the other I wasn't going to stop them.
But in some cases things can get iffy, when players who are good at every class are picked early in drafts and then captains later realize they would be more suited playing another class. For example, a few days ago I first picked b4nny in a pug on scout and then once a few rounds of picking went by, realized that there were more good scouts than good roamers so I picked a scout main (vari) on roamer and asked if they were cool switching. There are lots of mind games and things that went into my first pick and had I predicted that all of the good soldiers would be gone I would have done my first pick differently, but since there were 3 mainclassing invite scouts (cyzer slemnish and b4nny) I figured it would be smart to get at least one of them (so as not to let the second pick get the other two and win the pug off of his scouts carrying). Also some people like rick often add up to multiple classes and prefer to play one but think they may not be picked if they only add on that class.