In 2009, I had a rather controversial talk about this on FOSDEM in the XMPP DevRoom.
The main point is: even if your client did support message editing, you can make no guarantees (unless you're building an internal network), that the receiving party will support it.
Here's the trick with skype: Skype had only one, single client from one, single vendor, called Skype, until very recently, when they started to distribute SkypeSDK, which allowed car manufacturers, TV manufacturers etc to build Skype clients. Yet even those clients are built on Skype SDK, and therefore, most of them has to support this editing.
With XMPP, in case you're on the public Jabber infrastructure, you can't be sure what kind of servers or clients do your mates use, and there's only one thing you could be sure of: that they support RFC 3920 and 3921. (Yes, I know these are the old standards: you can't be sure they support the new ones).
It'd be easy to say that most of your contacts will be GTalk contacts using GMail, therefore whatever isn't supported by GTalk in GMail is virtually non-existent in the Jabber world, this includes message editing. Or, you could say, that there are some Pidgin / Telepathy users you talk a lot, but perhaps even file sending is still out-of-bounds for you.
On intranet, the situation differs, you can install a client which supports this feature (Swift was mentioned in the comments for the previous answer), but even then, you can be only sure about the internal communication network.
It turns out, sometimes it's better to build an IM service from scratch than to use XMPP.