I always love this assumption... When I train Danzan Ryu, I spar with other Danzan Ryu guys.... but I also spar with the new guys who just started. Who are the new guys? Some have no clue how to fight. Some know how to fight on the street. Some have taken other martial arts styles. The truth is, you don't really know what the new guy knows or how he is going to respond. In fact, the experienced guy sparring with the new guy has to factor in a lot of things. First he has to deal with whatever it is that the new guy knows: nothing, street fighting, boxing, judo, wrestling... Second, he needs to handle it in a way that does not hurt the new guy and does not get himself hurt. Third, he has to do all this while using the art that he is studying. (if you are in karate class, and the new guy goes for a Judo throw, because he is a black belt in Judo, and you counter and use a double leg takedown... that looks bad for the karate class...)
I have sparred with the new guys as part of Danzan Ryu, Karate, and MMA classes. The new guys have run the entire spectrum of never fought at all ever, to beat the tar out of people in bar room brawl guys, to high school and college wrestlers and people with more years of martial arts training then I have. I would like to say I whipped them all.... but we know that didn't happen. However, I did learn a ton and got to learn how to adapt quickly.