Most game designers are not psychics.* Offering different levels of challenges makes your game available to a larger number of players across a spectrum of aptitude, experience, and even mood. Many games that do not have an explicit difficulty selection option instead have different difficulty protagonist and antagonist choices. This is especially true in strategic and tactical war games for instance where you can choose your side and a scenario. It is also true of most CRPGs, there are easy classes and hard classes even in old style real roguelikes.
Adaptive difficulty that makes the game harder is quite interesting. Making it easier seems counterproductive, outside of explicit player choice via selecting easy difficulty mode, I expect a game to get progressively harder. End game and late game play can easily become boring if the game doesn't become more difficult. The trouble is, again most designers are not psychics. It is a complete crapshoot as to whether or not the designer's initial choice in rate of progression is satisfactory.
As to meta-gaming an adaptive difficulty implementation.. well if that floats your boat have fun. Personally I don't see the point for stand alone crpgs or turn based tactical where you are only competing ultimately against yourself. However, games are all about wasting time in amusing ways, so if it amuses someone, great!
As far as the designer having some holy vision and set in stone rule set, that's a recipe for failure. A vision is a plan, great to have one, needed even, but as in the nature of all plans it will not survive contact with the enemy unscathed. A vision and the ruleset which bounds its expression, must both be adaptable to the realities of the presentation environment and the audience if it is to have any lasting presence. Adaptive difficulty seems to be a way to help automate that necessary adaption for the designer and let him or her get on with adding more depth or creating a new experience.
*Can't prove that none of them are, there are a few that may be, then again perhaps its another type of psych behavior: psychotic.