Because most people are bad at math and can't do it the way their 3rd grade teacher taught them anyway. It should always just be round up or down, but without any exceptions.
Since WOTC has made the scores meaningless there's no point in keeping them around. The bell curve bonuses that TSR-era ability scores had made more sense though, and they should bring that back. Then they wouldn't be so pointless.
Definitely do not have them play more than one PC. I'd probably add one NPC just to help them along and bounce ideas off of when speaking in character, but they shouldn't really do all that much. Hold the torch for them and carry the bags of heavy stuff sort of things. I'd just make up some...
I'm not against homebrewing D&D. I do it all the time. But if another game already has RAW something I want to homebrew into D&D, it's perfectly reasonable to consider just playing that other game instead. Claiming so doesn't make you a RAW purist.