Maybe the writer for IM3 forgot that the ARC reactor is what powers the suits... or forgot to explain why this new generation of suits couldn't be (even if they ran out of power, they should still be able to work from his power source).

I remember that the whole ending (plot device) of the first movie was centered around his older ARC reactor draining, to only give him a limited amount of time in the suit. If the suit had its own power supply, the first movie ending made no sense. And you'd think that with "advancements in technology" the 3rd movie suits would still be able to use his power-source in addition to its own. (but then, we wouldn't have a significant plot-device in the 3rd movie)


As for the comments about Tony being out of his suit too much this time, this movie would have cost the film company a lot more for RD jr, so they had to get as much screen-time of his face as possible, to make the most of their investment. They don't need RD jr on set to film the suit, making him an expensive element if his face is only on-screen for 20 minutes of the film.