bingo! noticed in certain conditions filters were creating artifacts, hence laying off them, it seems to create reflections likely due to the light bouncing off one piece of glass onto the other. Other times I'd notice odd softness in low light.If the OP is shooting bands in bars, the last thing he needs is a flat piece of glass in front of the lens that does absolutely nothing except cause ghosting of every bright light source in an otherwise dark frame.
Not a bar, but the value of a protective filter depends on shooting conditions, in terms of both environment and illumination.
This is from a long, long time ago in a galaxy far, far away with a Rebel XTi and an EF-S 18-55mm f/3.5-5.6 (non-IS) that was absolutely horrible. It had a soft spot in the mid-frame directly left of center that was soft, even when other areas of the frame both further and closer to the camera than the part of the frame at that spot were sharper. But that's neither here nor there. What this image illustrates is what happens with flat filter in front of a lens when there are bright light sources in an otherwise dark frame.