Are the iPhone screens strong enough without protection? Can withstand the rare drops once in a while? I have a thick case though.

OK will get a proper one.

I bought one and installed it myself but ended up having dust spots and later I messed it up while trying to fix it. Got one more installed at a shop but one edge doesn't stick to the screen fully cos of the curve. Less than a mm but everytime I turn on the phone I automatically focus on that.

Many of you don't use them. I'll give it a try if my current SP is damaged.