Being the 48th most commonly used word in US English is a little misleading in this case, though. Verbs such as "get", common as they are, are rarely used to begin a thought.
The parameter count is enforced for each command. For GET, it needs exactly one parameter or it won't be treated as a command. So from your examples above, the longer lines would tweet fine.
I agree with treetrouble, however, this was a really bad engineering decision. They should have come up with a tweet prefix to mean "command", some symbol that would not ever start a real tweet.
Well it doesn't have to be a symbol, just something that would not be likely to start a real tweet.
Just as an example let's say "q", followed by a space, followed by the command with the appropriate number of parameters, e.g., "q get username". That would allow otherwise-reasonable tweets that are currently being mis-interpreted ("get better" vs. "q get better").
That's just an example. There are other ways around the problem too without making things too inconvenient on a smartphone user. And sure it's possible someone somewhere will want to write a real tweet that says "q get better", but that's far far less likely than someone somewhere wanting to tweet "get better". And the point is that allowing a non-trivial portion of the tweet-space overlap with the command-space is a bad decision. A trivial overlap (like "q get better") would have been a much better decision.
"Get" in its present-tense for is very difficult to use to start a sentence. It sounds highly idiomatic ("wrong") not to precede it with either a subject ("I get six floppies for a dollar at the thrift store."), or to recede to the infinitive ("To get six floppies for a dollar is a very good deal."). Using a gerund makes it easier, but then you're not dealing with the plain "get" anymore.
The second person imperative always implies the subject (at least since we stopped saying thee, and even then the subject and verb were inverted, as it remains in the first person plural imperative). The first sentence in the second example is a complete sentence; only the second is a fragment.
If you see an imperative "get" with an apparent subject, it's probably a topic rather than a subject:
"Paul, get your boots on and get going!"
You are speaking to Paul:
"Get your boots on and get going!"
You use the topic to designate or call attention; it's not the subject of the sentence.
Well there aren't many convenient choices on a phone. # is used in twitter, * is often used by the carrier. Might have been better to prefix with just "z".
You're right that it wouldn't be the 48th most common word to begin a sentence. Still, it's not uncommon enough to warrant not prefixing it. This goes for "stats" as well...
