First, my references for this:
MDN apparently indicates that this is a HTML5 feature , which makes me concerned that not all browsers will support this. I then tested a demo  in IE8, and found that while the <input> behaved as expected, the <textarea> did not. So for some browsers, our Cell will need event handlers specific to this, trying to make the behavior consistent.
Back to the demo  - this page indicates that Opera, which GXT supports, has an alternative implementation. Apparently instead of preventing the user from typing, an in-browser tooltip appears, warning the user that the entered (or pasted! there is another test case to have to handle, in all browsers...) text is too long and should be cut down.
So we're not avoiding this because we're lazy - we're targeting the bugs that are affecting users, and adding the features that won't break existing builds (at least for bugfix releases, 3.1 again may change some APIs), and we've got to take much more care with all browsers and all features than the answers provided in this section. If someone puts together a full solution dealing with all the addressed concerns (that I came up with testing two browsers and writing this in 20 minutes) and would like to sign a CLA to have it contributed, that is more than reasonable - it is welcomed!
Commonplace? Yes. Easy? No. Avoiding? No. No one wins if we just pepper in new features halfbaked and poorly executed, especially while we have a community that can present workarounds, as has happened here.