In general, extended warranty plans are a healthy-sized profit center for the company offering them. This implies that unless a product turns out to be less reliable than predicted, on average (mean average) customers will be better off paying for their own repairs without the warranty. However, if one can't afford to self-insure for abnormal or even normal deviations from the average, something like AppleCare will lessen the downside financial risks.
Thus, I suggest (and buy) AppleCare only if an Apple product seems to be getting numbers of reports of below average reliability (e.g. the Duo 2xx and PowerBook 5x00 product lines). For other Apple products, maybe put the money in a savings account instead, and there is a good likelyhood that you'll end up ahead after a decade or two of Apple products.
Is it worth taking it in? Absolutely! Apple have legendary customer service and will likely fix things they have absolutely no legal obligation to fix.
The power cord is absolutely covered. Apple will replace it free of charge under your warranty. As for the weathered keyboard key, they may likely do the same as that shouldn't be happening and a replacement key (which they will likely have a plentiful supply of in the back) costs next to nothing and is trivial to swap.
As for the scuffing on the case, they may shy away from that as that is a) a considerable amount of work to do and b) not cheap. But press them and you'll find that they may just acquiesce. The unit should after all retain its coating and they may tell you it's a "one time cosmetic replacement," but will still do the servicing.
I have personally had them replace my iPhone even though I couldn't reproduce the problem I had reported. My friend had his replaced, free of charge, after he dropped it and admitted to such when bringing it in (he is not alone here, as I have had other friends receive the same treatment). They also overnighted a new battery when my MacBook Pro was 6 months out of warranty!
I always encourage people to make use of their warranty. Apple's customer service is bar none the best in the world. Truly. They will make you feel like you own part of the company. I cannot express here how good it really is.
While you may not find precedent for them to fix any of those issues, you'll soon find that you won't need to. Apple works to make you happy, and not just to meet some legal requirement.
Best Answer
I purchased my Applecare from China (while I was living there), my MacBook came from Canada, and now I live in New Zealand, where I can still use my Applecare.
Mobile products (like MacBooks) have a worldwide warranty, and that extends to purchases for Applecare as well. Just make sure you are purchasing from a trustworthy seller and you should be set.
Good luck.