There are tons of good planes without AS3X, but for alot of people, the AS3X planes fly so well and are so convenient, they dont want to spend money on something else. Most people cant buy every great plane, but if you want a couple great flying micros, they'll go for the AS3X ones. If it was a crutch then flying a 100 inch wingspan plane is a crutch because its so large and stable
AS3X does a great job of limiting movement, rather than stopping it(except in the heading hold applications on helis). AS3X planes often still need a little trim, sometimes might still have a little coupling.