I consider it a mandatory variant. It takes away some chance of any one race when you play in a season. When only playing one race players that are in the back half can get bored since there is relatively little chance of winning. When you start scoring points for finishing, then you have a reason to fight for one more place and some more points.
The league I am in only awards points for finishing races. Even after four races we already have team rivalries.
I did a league recently and I found a few house rules can make things more fun.
You want as many people in your league as possible, so you want to make it as fair and balanced as you can. First off, not everyone will be able to make it every week. You can offset this problem by saying that players can drop their worst 1 or 2 results.(depending on how many races you have in your season).
Qualifying. Instead of the normal rules, we did the following... At the beginning of each race, each player rolls the black dice and adds their finishing position from the previous race. Highest qualifies first, etc. This means that if you get unlucky for a whole race (which can happen) and finish last, all is not lost. Lastly, we have a rule that you only retire if you run out of engine points (we also run the over-revving rule). Anything else and you spin. We do this because a spin is pretty devastating as it is. It's enough of a penalty for someone who has been reckless and retiring is too much of a penalty for someone who has been unfortunate. If keeps people in the game, which is important over a season.