@InnueThere aren't any problem with doing a little math. Hell, it's only a degree more difficult than adding or subtracting values.
The problem is with the indications.
Fixed point gains give us a nice and predictable system. Yours would be almost like a mess in this aspect.
It'd introduce complex strategies which may not discriminate against new players but in return would pretty much bane players who participated in more matches in the beginning. They'll be around the top and earn meager point amounts while a "newb" can technically catch up to them in two matches at worst.
It may keep motivating people but I strongly feel you haven't thought about the full implications of this system.
Then you aren't creating a system necessarily indicative of any skill. You are creating a 'who has fought the most with a semi-decent winrate' system. You need a system with some kind of scaling or tethering. It really isn't an option.
There are no complex strategies to be executed. The only people that could 'game' the system would be the lower ranked people who could pick fights with much higher ranked individuals for large point gains (which is precisely how it is designed to work). However, since they aren't expected to win those matches, then it shouldn't be a concern.
I've used a system with variable points gains for almost a decade for my own stuff with great success. I'm not suggesting it out of a lack of thought or lack of experience. I know it works.
EDIT: Additional rules may be needed in terms of ability to accept/decline matches, but that is a separate discussion to the actual points.