Deleting cookies logs you out of sites that you have set to "always stay logged in" to, which can be a minor pain even if you don't delete saved passwords. That said, when I worked in tech support for a web based application, 90% of the calls I got consisted of
Caller: "Such and such isn't working."
Me: "Have you cleared your cache and deleted cookies?"
Caller: "No, how do I do that?"
Me: Explains
Caller: "Okay, cool, it works now, thanks!"
Me: "Fuck you moron, stop wasting my time, I could have been surfing the web just then."
So yeah, I'd recommend doing it even if it is annoying. It will usually fix your problem, and that's worth it. That being said, I just cleared cookies/cache and I still can't get Gabbly to load...