Yep, it has absolutely nothing to do with growing larger antlers and everything to do with allowing bucks to age. That's why they give you an extra tag that 95% of the time can only be used on a yearling buck. Heck, I can now kill 2 yearling bucks legally if I want to. Sounds like you have it figured out.
I'm not in an AR county so I don't know whether they are good or bad, I have no experience in those counties.
I do know I'd be frustrated if I were constantly seeing 3 1/2 to 4 1/2 old deer that didn't meet ARs. Because in my 6 years of hunting in Fisher County, I can honestly say I've only seen 1 deer from the blind that was over 5 1/2 and I happened to shoot him. In addition I only get pictures of very few that I believe are 4 1/2 and older.
So I think it is a bit much to expect the public to only shoot 5 1/2 year old deer and older. I tend to believe going by width only will allow deer to get older, but will also result in people shooting the 2 1/2 and 3 1/2 year old deer with the better genetics. At least that is what the results were in Louisiana when they implemented antler restrictions. Horn length at each age group actually decreased.
But I seriously doubt they "WANT" you to kill yearling spikes. I tend to believe the purpose of them allowing you to shoot spikes is for when people discover that the doe they shot had small spikes that broke the skin. I think the intention of the doe/spike season is for accidents, not for people to go out and shoot yearling spikes.