Well you could test on a lot of different devices without designating them as developer devices(Ipad, ipad2, iphone 3, iphone4, iphone 4s, I have all these devices, but I use them, and don't want to make them developer devices). You my have thousands of dollars to waste on that or you could just spend $99 for MobiOne.
Also webGL is not the only problem with mobile, in fact I don't think it is the main problem. I believe loading the entire game on bootup to the phones RAM is the problem. My game that is on the Chrome Store Maze Manor was created to be a mobile app, but because the game dumps all the textures from over 60 levels into the RAM on bootup most mobiles can't handle it. Being able to easily test different devices to see which work is handy. Also the preview to mobile is not an accurate measure of performance. My game gets a 5x speed boost after it runs through appmobi and gets converted to an app.