Tweets can be and are deleted. Archives make sense in this case.
In deleted cases, yes, but not in advance. We love archives (and archive.org most of all), but archive links shouldn't be used as submission URLs when an original is available. For one thing, it's important for the original domain to be displayed.
Of course it's always fine to link to an archived copy from the comments, and we intend eventually to build software support for this.
To add to this, since it linked to the archived version I thought it was taken down for some reason. Giving the original source makes it clear it's still up.
That's totally logical and something I didn't really think about. It's amazing how much information can be implied from context and juxtaposition.