Slow updates do hurt sales. And why? Because the rate at which the webcomic is being updated represents the rate at which the print comic is being assembled. If the webcomic is taking forever to progress, the comic is taking forever to get made. If it takes forever to make a comic, it takes forever to put that comic on the market, and it takes forever for you to make any money from that comic. And until the comic is selling, every day is a loss of profit, what with having to pay for web maintenance, bandwidth, advertising, going to cons, the artist's salary, etc.

Now, I believe the site is being maintained with the profits from the whole "tool" thing, but think about it. People are only going to pay for membership on a site that rarely updates for so long. Heck, if this thread and Erfwiki are any indicator, people are already starting to drift away, and that's less people paying membership fees.

If you are making a webcomic, or a comic, or a book, or a graphic novel, or anything for profit, inconsistencies and delays equal profit loss.