I was mentioning this problem here about one year ago, and Adam told me
that this was the correct behaviour as documented by Apple:
**
To be valid, the receiver must have a formatter capable of converting
the object to and from its textual representation.
**
I don't like this either, because if you intend to use string values,
you need a dummy formatter that just passes the value. I couldn't
really believe Cocoa's behaviour was like this, but I don't have access
to a Cocoa machine, so I couldn't verify it.