When it comes to that, if the other people are following the same rule the aren't bringing in an accumulated amount of stale sweat, funk, and festering germ/bacteria culture in with them when they roll (and subsequently sharing said stale sweat, funk and germ/bacteria culture with me). Yeah, I'm going to be sweaty at the end of class. That's why I shower afterwards.
And yes, some people do wipe down right after rolling. That's the reason why these exist:
No one is saying that there is some sort of minimum number of gis that is required to start training in BJJ. Just keep what you have clean. If you live in a warm and arid place where you can do that, well I guess that's an option to keep the gi usable. Just realize that having a "usable" gi is very much different than having a "clean" gi. Also, if you live some place like western Washington state where it rains 80% of the year doing the "sun dry" thing isn't an option.
People state things like this because it's not about being "rude" or a hedging the varying opinion about what is considered rude. You come in with a gi that isn't clean and you are creating a situation where your training partners health (and ability to train) is at risk. Period. Just saying "well you're going to get sweaty/dirty anyways" isn't a realistic, mature or acceptable mindset to have about it either. If that's the case, you might as well be asking me to willingly roll with a leper.
I ain't doing that. If I tell you I'm not going to and you find that offensive, well (in this instance) that's on you.