There might exist some other fingerprints that can lead to the knowledge of what browser you're using (the existence or non-existence of some feature, etc). If your User-Agent contradicts those others fingerprints, you become very easy to identify. It's really difficult to consistently pretend having a given configuration unless you are, well, actually running this configuration.
More generally, the more you use technologies to protect your privacy, the more you increase your fingerprint entropy, that is, the more it's easy to identify you.[1]
I wouldn't advice those "spoofing" tools, but rather to use the browsers' settings at the maximum of their capabilities (disable third-part cookies, delete cookies when closing the session, set Flash to 'Ask to activate', etc) and just the minimum extensions set to block third-part requests in a first-place (Disconnect, etc).