'Yours Faithfully, Yours Sincerely or Thanks' - When to use?

Yours faithfully/sincerely always strikes me as being the wrong way around. Surely you can only be faithful to someone you know. However, you can be sincere whether you know the person or not. I suppose it means that my letter is faithful to the truth, but that's not the predominant usage of faithful these days.

I always get a bit edgy when 'they' say that this is the way that things 'should' be done. Why? Who made you boss of the English language? The reason that English lives and grows as a language is because it changes. Otherwise we'd all be talking like Chaucer. To my mind there are onlt two rules to English usage: write what you mean in as short and clear a way as possible, and never use multiple exclamation marks at the end of a sentence. (In fact, never use exclamation marks, you probably don't need it.)

Now, will somebody please point out all the spelling and gramatical errors I've just made in this rant.

This sounds a little odd, use one or the other. If it is very formal, use Yours sincerely.
If you are unsure, Kind Regards will work in almost all cases.

(I'm a native English speaker)

As an expeditor, at work I spend probably 50% of my time writing to businesses.

Correspondence is about 90%  email so the endings I recieve are quite curious. By and large however, it's almost always 'Kind regards' which is ultimately a friendly ending. If replying to a response that has been helpful to me, I will quite often end with 'Many thanks'.

I wouldn't use that in a letter though, in fact so long ago was it since I last wrote a letter (as against writing an email) I couldn't remember how to end it so found this forum for advice.

So why wouldn't I end a letter with 'Kind regards'? I think it's the culture of email in that they were invented to be brief or chatty and not intended to be used as a business tool. That they (emails) have now mainly superceded letters, certainly in my line of work, probably means the end of Dear Sir, Yours Faithfully etc. apart from those sent by your bank advising how much you are overdrawn once more.

Yours.......

(Now, how do I end this message correctly? )

On a slightly different tack, which do you use when you address the letter "Dear Colleague". It is impersonal in that it is not addressed by name but you may well know the colleague. I have just sent a batch of letter to a group of colleagues who I know very well but for the sake of ease they were all addressed "Dear Colleague".

I can't see any of them being offended (if they even know whether I have been incorrect) but was interested whether I was correct when i used "yours Sincerely".