For me, the cover stitch -- used to hem tee shirt, swim suit, and other stretchy fabrics -- was a must. Only the more expensive air-threading machines offer this. If you want one of the basic air-threading machines, and don't need the cover stitch, that's great. Should you ever need it, you can buy a stand alone cover stitch machine, and use it just when needed. It need not be air threaded, as with 2 needles and one looper thread, all of which are relatively easy to learn -- and all of us will tell you that you can "tie on" a new color of thread, and pull it through, up to the eye of the needle.
The Baby Lock machine I use at work is wonderful, with the air threading! but -- there is no way to thread it if this system is not working; ours did have a build up of lint from threads over time, and needed a service call to clean it out -- we use it extensively at work to do the most beautiful rolled hems on many craft items we make for
our shop.