Magick Archer, Mystic Warrior, Assassin and Ranger in particular require training in other classes to reach their full potential.
A gamer who doesn't know this will be dealing next to no damage and will be getting killed a lot.
If your build is poor, you flat out cannot progress through the game (even on easy mode).
So I'd argue that this is "need to know" type information.
I disagree - I find the system quite forgiving and flexible. If you stay with a certain class (like I would assume a lot of gamers will, they find one they like and to hell with the rest), you'll get stat gains relevant to your class. I didn't know the growths for each class until way after 100 IIRC, when it was pretty much too late to do anything about it. I'd stuck with magic classes while my pawn stuck with physical ones, because that's what I liked. Smooth sailing.
If you keep switching classes, the pre-100 gains are all good, even a mage gets +2 physical attack per level. There are only a few +1s and no 0s. So you're getting something in each stat regardless of what class you are. You'd have to be switching classes a lot - A LOT - to get a sub-par build all around. I'm still not convinced this is possible.
Sure, you can end with a less-than-ideal or overly-lopsided Arisen (like mine), but I can honestly say I've never heard of someone unable to progress due to their build. Switching to a physical class after ages of leveling as a magical one is tough, and yeah, you can find yourself doing little damage, but... if you spent that much time as a mage/sorcerer, you always have that to fall back on if you can't progress as a fighter. Or you can upgrade your gear.
Folks who like to "optimize" their builds are mostly doing it to maximize damage against the Ur-Dragon, from what I've seen. (And thanks, people out there that do this. I help out, but man, that life bar doesn't move.) The main game is pretty fair for any build.
edit for current team: