My place won't allow noobs to spar for a couple of months. These are the reasons from what I've gathered;
1. Noobs lack control, first time they haul off and throw a bomb at one of the experienced guys, the experienced guy is likely to take it personal and knock them on their ass.
2. Noobs lack proper technique, if we let them spar right away they are more likely to develope bad habits that will have to be corrected later.
Noobs do pad work and conditioning, but at the school I train at they are allowed to participate in "timing drills" These drills allow very light contact at a controlled speed in an environment supervised by the coach. In this way they are forced to focus on technique while trading shots with another student. Once the coach feels they are far enough along to come to FC sparring class he lets them know.
I prefer it when there's a night specifically for fighters. We have a sort of small gym, so when we have to run inside, I don't like being impeded by new guys who can't push themselves.
Our policy is similar in that people need to learn the basics before they can fight. If an experienced MT fighter shows up he can jump right in, but there really isn't a point if its a new guy out there flailing like a fish.