Gilad Bracha writes that the JVM shouldn't get continuation support and gives some thoughts to continuation based web servers as well.
Summary: In the short term, continuation based web servers are very nice. Seaside is the nicest one I've seen. But ultimately they are just a phase, and we can already see how we will outgrow that phase. Since continuation-based servers won't be all that significant in the long term, and given the huge lead times and costs of adding continuations to the JVM, it makes little sense to support them.
In a recent post by Avi, author of Seaside, to the Seaside mailing list, he comments that DabbleDB (which is written in Seaside) actually makes very little use of the continuation based elements of the framework:
As a data point, it's certainly interesting to note that Dabble DB makes very little use of call/answer - the only major uses are when importing and when processing payments. We were intentionally going for a very modeless UI style, so this shouldn't be too surprising. The question is whether this kind of modeless web app will become more common... it seems like it probably will. If so, then yes, the continuation aspects of Seaside will become less interesting, and the callback aspects will become more interesting...
I've found similar in web applications I've been writing lately. Less use of the return from the continuation but making heavy use of the ability to register functions to be called on button clicks, etc (the callback aspect). But when you need the use of the continuation returning for workflow it's very very useful to have it.