I find it so interesting how we have taken the exact same word, applied similar context, and yet have vastly differing reactions to it.
Now, perhaps I'm just a dumb American, but to me, spastic is just acting in a rapid, clumsy manner. Arms flailing about, tripping over things, is the image that comes to mind. It's not just being clumsy, but the extreme speed at which it occurs. That, or extreme hyperactivity. Kid hopped up on way too much caffeine, for instance. A rather silly image, that has come to be used as a minor insult, usually for self deprecation. Very similar to nerd, in that respect. Was somewhat offensive, now not even the tiniest bit offensive, even to the perpetually offended.
However, in the UK, it is one of the most insulting words possible. For fellow dumb Americans, note how Scape felt the need to self-censor, even when using the word to say don't use it again. Over there, it is second only to retard in terms of insulting for disabled folks, and with very similar meaning.
To put this in proper context for both sides, I am going to apply the same treatment to another word that offends everybody, the word that I would never use outside of discussing the word itself, and even then would seek to avoid: nigger. I sat for a solid two minutes steeling myself before I wrote that word. That is how offensive it is in my culture. Even now, I am contemplating scrapping this entire analogy, but it is the only word I can think of that is heavy enough to shock people into thinking about this.
So. Let's say that there is a culture that uses the word nigger. In that context, it is a word used for black people. Nothing more, nothing less. Sometimes, it is even used as the color black itself, in very specific contexts. They use nigger as greetings, use it in products, have it used on the television all the time. How angry would you be if you saw this culture? You would be furious! No, that word does not mean what you think it does! Stop using it, now and forever!
"But why? It's not offensive! Our definition is not yours! Why should we be offended for you?"
And here is the rub. It is not offensive to them. Not at all. But for us, it is. So, who is in the right? Should the other culture censor the word in recognition of a definition they do not share, or should we just accept that it means something else over there and leave them alone? I, personally, while feeling uncomfortable, would say they have the right to their definition. The word is not inherently offensive. String those letters together, not every living thing is repulsed by the sound. What makes a word offensive is the context and definition we give it. I would even accept it's use on, say, a posting board if used by those people. That does not mean I would cringe any less, and would probably contemplate telling them that they should stop using it for my emotional state. But I would not, because respecting differing cultures is important, and if we can make more words less offensive, the better.
Now, if you excuse me, I have to wash my mouth out with soap.