If the lens surfaces are very worn, the most noticable problem you will have is a diffusion effect or flare around strong highlights. I had an Optar like this once and had to return it.

Many older lenses show a bit of a halo around strong highlights anyway due to spherical aberration (it can actually be a nice effect), but it has a somewhat different quality than the effect of diffusion or flare, as the image should still be sharp if the glass is clean, and it should sharpen up as you stop down.

If you find you can't make a sharp image at any aperture, get another lens.