According to Heinlein there were two reasons for his using psuedonyms. First was that Campbell did not want to have more than one Heinlein story per issue in his magazine... and Heinlein was so prolific that he often had two or three stories published in the same issue... just under different names. Secondly, even the great Heinlein was occassionally got rejection notices from Campbell and those not-up-to-snuff stories got published elsewhere under a psuedonym.
That's interesting. As I edited the short stories list (post#88) I wondered why a great writer like Heinlein would use pseudonyms.