All the eggs in one basket

Would you seriously trust a 3rd party computer in this day and age? I have a complete aversion to entering usernames and passwords into computer I do not own. You have absolutly no idea what's installed on the machine, software or hardware (I dont buy the personal bootable CD as a serious privicy option given you have no control over the hardware). It always amazes me when I go on holiday to remote places, when you get to a town people run into the nearest internet cafe to check their hotmail, I bet most of these machines are running hacked versions of windows, with no security, and probably full of spyware...

Example 1:
you have an email service. You dislike the quality of the service and you want to move to a new one. How do you bulk transfer your messages from one service to the other one via webmail? You don't.

Example 2:
you receive email on 2 different email addresses on different services. Say you have an office email and a private email. Now your colleagues know both and by mistake often send you private emails on your office address or the other way round. How do you organize your email (e.g. moving private messages in the right place) via webmail? You don't.

Example 3:
You do not trust your provider and you want to back up your email periodically on your local system. Or you do not get "infinite" space from your provider, so you need to clean up things periodically, but you also want to store somewhere your old email. How you bulk transfer messages from the server to local storage via webmail? You don't.

Example 4:
You have 5 different email addresses from different providers because you have many affiliations. How do you keep them under control all from within one window with webmail? You don't. How do you avoid having to learn to use 5 different interfaces with webmail? You don't.

So the conclusion is that webmail is typically easier to deploy (and thus a nice solution to have), but it can offer you only an extremely limited subset of what IMAP offers.

[By the way, this is why it is truly sad that some email providers such as gmail do not give you an IMAP access but only webmail or pop.]

Unfortunately when there is a possibility to chose between two options, one with severe limits and the other one without, most people not only will happily choose the first (that is legitimate), but will also implicitly pretend that having two options is useless complication.

The result is that rather than defending the freedom of choice (in this case suggesting that an IMAP client should be always installed/installable alongside a browser wherever needed), the best that is done is often to invoke hacks on the limited solution to overcome some of the limits. The problem is that often the limited solution is _by design_ hardly adaptable to new behaviours. And furthermore, by removing some of the limits often it becomes as complicated as the original unlimited one.

As a final remark, note that it is in fact quite easy to have anything running anywhere without installation. For instance think of vmplayer virtual appliances.

It's not a silver bullet, and it is a hammer only for those who view every problem as nail. Its deficiencies are too well known, and strange, more and more stuff tends to be done "over http".

Nope. More and more stuff is done "in browser", not "over http". Sure, sometimes http is used as replacement for ftp - but there are no problems: it works well for that. In most other cases the goal is not to do something "via http", it's to do something "in browser".

What's the reason ? Firewall/proxy piercing ? False and unfounded sense of security ?

Nope. Usability. When you have something "in browser" (your mail, your news, your documents and so on) you can always access them. If you try to use something else (SMTP, IMAP, NNTP, etc) - you need client. And quite often you just can not install that client (corporate policy) or you just have not done it due to laziness.

We have the whole IPv4, why limit ourselves to mere http ?

Because it's easier. It's logical next step: very few are installing OS today (that's Windows is king and Linux is lagging), the next step is that few people want to install anything unless they absolutely need to. If web-based solution works - it's preferable to client-based solution. Netscape creator were right in a sense when they said that in the future web-browser will be the king and OS will be significantly less relevant - they just said it 15-20 years too early...

The fact is: people are scared by client solutions. They know that installation of something on Windows can easily screw your system and on Linux you can only easily install something if it's in your distribution repository. Cleanup (if you don't like something) is often incomplete (both in Windows and Linux). But with Web - it's easy: visit web-site, try the thing, if it does not work adequately - forget about it.