No one should be using Ext. Ext has had a very shady history regarding licenses (http://en.wikipedia.org/wiki/ExtJS#License_History) and they have tried to stop developers from using their previous LGPL code citing some spurious interpretation of license.
Bottom line is, use jQuery or dojo or prototype if you want to avoid headaches.
The company behind Ext wants to have the cake and eat it too. They want people to adopt the code (hence the GPL) but they want people to pay for it too.
I don't see what Ext Core - which is released under the MIT license! - has to do with the GPL license of the Ext JS library.
Nobody has to pay for using Ext Core, so the only valid reason for not choosing it is a dislike for its syntax, internal workings or any other technical reason.
It's the same reason people are suspicious when Microsoft associates itself with open-source software. Past actions make people suspicious as to whether they can rely on them for support in the future, especially when it comes to something like frameworks where large code-bases rely on its continued existence and the vendor not pulling the plug, and kill any ill-will in the community.
Bottom line is, use jQuery or dojo or prototype if you want to avoid headaches.
The company behind Ext wants to have the cake and eat it too. They want people to adopt the code (hence the GPL) but they want people to pay for it too.