All good points. Why this number was chosen and not the other - the answer is simple: it came out from a brainstorming session among BPs. We took it upon ourselves to propose a single point in those recommendations. We could argue about exact numbers for ever and never come to any agreement - "analysis paralisys". Hence it sometimes helps to pick a reasonable number an run with it.
Also if you think it's terrible idea - just vote 'No'. We get more 'No' then "Yes' and put this issue to rest for once and for all.
I agree this doesn't resolve the issue of matching capable BPs to votes. It's a much bigger problem to solve that has to be solved outside of Blockchain.