Myself I've been using them (screen protectors) for sometime now. Even the most careful as you've found get scratches on their devices. Myself I started using a screen protector as the day to day use an rubbing against my face, pocket, case etc. were starting to cause a film on the screen in the small groove thus the reason I started using one. With the 7100's being more of a Nokia type screen I would deffinately recommend it. Honestly I forget its there until I'm reminded.
I bought the WriteRight microfilm covers. I think there were 12 for $14 at BestBuy. They are large, but the piece they are initially stuck to (out of the package) has a grid on it that makes it easy to trim them down. So it was very easy to trim down to fit the 7100 perfectly. They also came with an application device (but any credit card would work) and I was able to put it on without bubbles.