Interesting feature for sure. I switched to buying manga several years ago, as it was cheaper than DVD's and I could read them at work without catching flack for watching DVD's or videos. Plus I would not get the "Is that anime porn?" line from the ignorant masses that seem to populate the majority of the US Military, sigh.
I strayed from the point I'm afraid. Back to business. I suppose one could do a usage study to see if the majority of users prefer to view the content online, within their browser (read: youtube) or download it for viewing later. Both are feasible! But why go the extra mile and provide an in-browser reader? Revenue! Manga pages are usually monochrome with shades of grey, which equates to a great compression ratio, depending on the file format used. It takes time for a end-user to read through a page, depending on the type of reader they are. That idle time could be used to precache the next page, presenting a seamless reading experience to the user. For revenue one could integrate interstitial advertisements related to the content.
In closing, I know that I would use this feature if it ever panned out. I would enjoy reading online, and possibly downloading the files to be read on a portable device later, such as an iPAQ. What woudl be really interesting is a portable device manga reader, that addresses the problems of reading image based pages on a limited resolution screen.