Perhaps a quick review of all the types of bannered routes exist in usausb. Aside from Alternate and Business, I recall Truck, Bypass, and Spur. I think there is even Alternate Truck and Alternate Bypass. Seem to remember Connector as well. Am I missing any?
While the OP noted that alternate routes are typically longer (which is true), there are some business routes exceeding 30 miles as well as some alternate routes shorter than 2 miles.
The bannered routes that are in usausb were once called "special routes" by AASTHO (might still be). I could see how that would be justification enough to keep things as they are. I suppose the bottom line is what will be most useful to members of this site. As pointed out above, some users are interested in driving the mainline highways and don't care about bannered routes. When I'm in an area, I try to clinch as many of both as I can.