Note: this post is about people, not technology.
Let’s imagine for a moment that you’re following someone on Twitter that’s fairly popular in your niche – not a star, mind you, but someone with a couple thousand followers. Someone like Gary Bernhardt or Phil Sturgeon.
One fine day, the person you follow tweets something interesting, thought or discussion provoking, or perhaps incorrect, and you find yourself wanting to reply.
Now for the trick question: do you reply?
If you do, you might be one of tens (or hundreds) of people saying the same thing, effectively flooding and annoying the target. But if you don’t, your possibly unique and interesting feedback won’t get heard. It’s not that unique, you decide, and you weren’t asked directly, why bother…
So you can either be apathetic or troll. Your choice.
What if you’re on the receiving side – you’re the big fish and get hundreds of replies to every other tweet you send? You can either ignore them (except from the people you know and maybe follow yourself), effectively making Twitter a broadcast medium ala RSS, or actually try to strike conversation with all the people replying to you, wasting a lot of time.
So you either use Twitter as RSS (with a few notable exceptions) or waste a lot of time and probably get annoyed a lot to boot. Your choice.
Beyond a few thousand followers, Twitter just doesn’t scale for conversations. You can either reach a big crowd or actually talk to a small number of people, but not both.
Blogs, Facebook or Google+ actually don’t have this problem. Why? Because, when commenting on someone’s post, you can see all the other comments. It’s up to you to decide whether you’re contributing something meaningful or meetoo-ing.
On Twitter, there’s no way to do that.

Try clicking or tapping on a tweet. It will show an expanded view with replies. I use it exactly as you said, to check whether my comment is original.
Thanks Scott. I was under the impression it only showed a couple of (arbitrary) replies, but apparently a lot of people rely on it, so maybe my point is moot. Thanks for pointing it out!