I got the Vikuiti ARMR200, which makes a big deal about remaining sensitive for resistive touchscreens.
I applied it before booting for the first time so can't compare before & after, but I have NO complaints about the sensitivity or visibility of the screen now, and my mind is at rest about scratching.

OK but that doesn't answer your question: do I need one?
The way I saw it was this: without a protector, I would constantly worry about scratches. So yes, I needed one. The fact that you're asking the same question makes me suspect that you need one too .