Tiller extensions work. But there are a couple advantages to tabs:
I got a little better speed with tabs than with putting my weight forward (not sure why, I just did).
The ride is a little better further back.
I had more confidence in turns seated at the rear than in the middle.
Also, I didn't put a middle seat in my boat -- I cut U frames and put a cooler between them. Even strapped down, it wobbled a bit.
The tiller extension gets in the way sometimes.
It's nice to just pull start and go without having to move forward.
You can't make really sharp turns with an extension (well, not intentionally anyway).
Of course you can take the extension off easily, which makes it a lot more flexible than tabs.