Yes, they use AdBlock, but there's still a difference here, right now they're seeing the tweets of their peers in-order, with ads intermixed - which makes the ads trivial to filter out (bad for twitter, good for my friends).

In the future they might see whatever Twitter thinks is important to them, which might still be ads, but in the form of other peoples / companies tweets which just "happen" to end up in their "timelines".

Now, of course they can start blocking just about every company/user they don't care / like on twitter, but the effort might not be worth the gains at some point. And who knows, maybe you won't event be able to block some things from their algorithmic timeline, because that might defeat their monetization goals.

And yes, I realize that I might be reading a lot into Twitter's actions at the moment, but given how they treated their platform in the past year(s), all I'm seeing is that they want monetize it at all costs.

Not in most third party clients (e.g., Tweetbot for OS X). I've hardly used the web site for Twitter and don't really like it. I can't imagine the service without a good native client.