I understand what you mean but I don't agree, my friend:
A king could not recruit as many knights as he wanted. It depended on many factors, many of them have to do with feudalism. One disastrous battle for example could limit the amount of available knights for years to come ( 1300 in Flanders, 1215 in Bouvines...)
There ought to be gameplay differences:
- Strong feudalism means that the nobility have the wealth to raise many knights, to support you as their king. Also, upkeep of knights ought to be low when garrisoned in their own province, except when on campaign. The downside is that they can rebel more easily, and less wealth goes to your coffers. Also city rights are harmed by strong feudalism, which means less income / building options in towns.
@ Polycarpe
For your Low, Standard or Strong nobility influence , check the following list:
http://en.wikipedia.org/wiki/Feudal_land_tenure