In general, web applications are great because they are cross-platformable. But they are unable to take advantage of specific OS hooks because they have to be generic enough to work on multiple platforms. You'd probably lose the quick entry box among other things.
And if you are in a cellular dead zone and unable to contact the server (for example, going through an underground tunnel), you are SOL.
Omni Group would also have to rewrite their business model and enter into a web business model. Going into cloud computing isn't as easy as you'd think. Look at Apple. They've screwed up MobileMe, had a lot of hiccups with iCloud, and are still trying to iron out the Mac App Store.
We've also seen quite a few cloud based task management apps such as Astrid has gone belly up. So converting OmniFocus to a web app isn't a sure shot.
http://www.usatoday.com/story/tech/p...strid/2618141/
There are plenty of other cloud-based task management apps available with their own strengths and weaknesses. Why not give those a go first? At the moment, it doesn't look like Omni will be going full cloud computing in the near future. The closest they're getting is the OmniFocus syncing solution - not a full web app.