I would think it is because the desire is to have riders that are of a standard fit to be in the top league rather than filling a team with fodder to gerrymander the remaining places
Personally with a RS position in place I would say that the other reserve should actually have to be at least 4 points let alone 3 or even2! (and non Brits should not be allowedto drop below this figure)
The other point in this which I will put forward in a devil's advocate role (because I understand the attraction of wanting the best for your team)
This shows some of the reason why the sport in the state it is - despite all that has gone on, and the league all but falling apart, (not out of the woods yet) the first serious chat about team building involves how the regulations may be changed/manipulated in order to give a team an advantage
And we wonder how we got to this point.....