AS arguably offers the least worst of the BE fares, though for most flights they're still not a particularly good choice. On AS you get full EQM earning and keep your boarding spot. The occasional PC class seating isn't special - people on BE fares on UA also sometimes get E+ seats. If you don't have status and you're flying a cheap ticket on an RJ, they're not necessarily a bad buy - you're getting a window or aisle anyway, the change fee is probably more than the ticket price for a non-BE ticket, and you already aren't getting an upgrade. If you're MVPG or higher they don't make sense - with the free changes, the extra $30 for a ticket in main makes it very comparable to a full-fare changeable Y (minus the instant upgrades) of most other carriers.

In any case, none of the airlines really want you to buy them. It's there so they show up in the first page along with NK on the various search engines. At this point the airlines have so confused the low end fare restrictions that the search engines are useless (probably intent on the part of the airlines) and unless you're planning absolutely no changes and a $1 difference in price matters to you, you have to go to all the airline sites anyway to figure out the actual price for your combination of flights and baggage.