I agree with you on the Arduino IDE limitations, but what I don't get is all the browser/cloud-based part. The mbed platform has all that online stuff, and that's precisely why it's a pain to use.
simonbarker87 makes a great case for us on the comments below. i'd just like to add that it generally skips the (sometimes painful) process of installing and updating the hardware. everyone who has taught a workshop or class on Arduino and had to help about 10 people get set up probably knows what i'm talking about.