I do think you can run just about any cans on 70mW. And very well. For HD650, that is around 120dB Spl. How loud could you possibly want to go? You will have to look far and wide to find any portable amp that puts out actual undistorted 70mW, especially for less than around $300. For the money, 70mW is a reasonable tradeoff.
That said, have to agree, desktop always preferable. IF YOU ARE SEDENTARY. I mean what's wrong with excess power? Fills you with confidence. Like mutually ensured destruction, or 500 HP pickup trucks, or more megapixels. Nice to know its there, if you ever need it.
I move with my laptop from room to room. I hate power cords. So there is your usage scenario for portable amps.
I have a philosophical problem with plugging an iem into a dedicated amp. I know there are very good iem's. I know most would benefit from good, clean, abundant power. But then there's the principal of the matter. A man has to have his principals.
As to how much power a headphone needs, you can safely disregard Amir. He likes to see his eardrums flapping in the wind - makes him feel patriotic, I think.