"Really necessary"? I suppose not, but then I guess neither is an altimeter or an airspeed indicator. I mean, you can look out the window to see how high you are and and feel what the plane is doing, right? Real pilots don't need those wussy instruments.
All kidding aside, if you do everything right you don't need AOA, or a stall warning. But if you do get a little too slow, or a little sloppy, or try a little too hard to salvage a misjudged crosswind approach, it can be very helpful in pointing out your folly before the situation gets too bad.I've had it "nudge" me on the base leg when I was paying more attention to my position than my airspeed and got a little slower than I intended. That happened a few knots before the buffet, which is what I want. Maybe I'm just the only one who hasn't perfected flying a stabilized approach, I don't know.
Being a very low time pilot as I am, and as I am still developing my skills, I actually find it most useful when I'm a foot or three above the runway. The AOA audio tones get progressively faster to give me a good indication of where I'm at to make a nice smooth touchdown, just at the stall. In ground effect I don't feel the buffeting that I do when practicing at altitude. Maybe if I had a few hundred more hours I wouldn't find it so useful.
The stall warning would squawk far too early and far too loudly. I know both could be adjusted, but in less time than it would have taken to adjust the tone and the angle of the switch I had AoA installed. AoA is a progressive and much more useful indication than the off/on nature of the stall warning switch.