It is very important.

I understand the argument but you might as well not have practice if you cant have some level (more than 15 minutes) of full contact during the week. Preseason is even more important to develop some level of tissue toughness. I always has some bruising on my arms during preseason that went away during the season. This rule would mean I would get that type of bruising after each game.

I'd be curious on two things
1) who came up with the time restrictions; and
2) how would you enforce it? Does 15 minutes mean a team or an individual. If you mean as a team you are going to get kids hurt because coaches are naturally going to have thier starters with the contact time over backups.