I Wonder why rules are thought to be superior as letting the coach call his plays. It seems to me that his play calling is less predictable than rules, then more difficult to gameplan against. Wrong ?

There are situations where you would want a certain call even if it's predictable and the AI doesn't handle many of those very well. Such as being down 14 with like 3 minutes left and it's 4th down. If you're in FG range the AI might kick a FG. Or in the 4th quarter when you know your opponent is going to be pass heavy because they are playing catch up, in this situation you may want to key the pass and have more DBs on the field but the AI would instead line up in neutral play calls or possible key running plays on 1st and 2nd down regardless of situation.

I get those situations. But there are players who seem to have a lot of rules : is it really better ?

There are a LOT of those types of situations. For instance, I recently had the ball on my opponent's 11 yard with 4 seconds to go, down 4 points...and my QB threw a 1 yard pass to my FB as time expired. You can be sure I now have a rule covering that situation. I've got rules to cover several clock management situations as well as rules that respond to other game situations. And I find more and more needs for rules at least every other game or so. Personally, I don't think you can compete with the really good owners without them.

Hum... seems i have to dig a little more into it. Lot of work ahead :/

This game is the deepest one of its type I've played. I've been here over 6 months and only recently started looking at Rules. Still lots and lots to learn.

I am definitely in the camp that believes rules (particularly on defense) are way better than allowing the AI to govern personnel, formation and play calls. I tend to give the AI more freedom with the offense because I prefer more unpredictability on offense. Nevertheless, I have rules for protecting a lead and trying to overcome a huge deficit.

My overall record after installing a comprehensive set of rules is 97-8. I have to admit that I still tinker with them between each game.

I have been known to strip out most of my rules for regular season games (with exception of time management and inside opponent's 5 yard line rules which pretty much all stay) and then put them back in, or create new ones for the playoffs.
That said, I usually only create a rule when I notice I got burned by something in particular.