Pseudonyms incur a substantial publication penalty. They're harder to market in this day and age. If you're publishing in a genre where pseudonyms are typical (erotica, etc) your market potential is already curtailed and it's less of a big deal. but if you'r'e going up in front of a real publisher, a pseudonym is a great way to ensure 50 cents on the dollar or less. Source: my agent
It builds a wall between you and your fans. Nowadays there's no marketing budget whatsoever so you need to be the one with the livejournal, the book page, the tour, the radio station call-ins, etc and if you aren't "you" the whole affair comes off as forced and fake and nobody wants to hear from you. Whatever reason you have for writing under a pseudonym, your fans will assume it's because you're a furry with a sexual attraction to tupperware and an arrest record a mile long for sexual assault on farm animals.