To Summarise

This is an attempt to summarise the widely-held opinions of photographers in general.
I believe this is a fair distillation of views.

• Adding any filter risks adding lens flare and reducing sharpness
• The effect of most filters can be reproduced in post processing
• The only filter which cannot be adequately reproduced in post is a polarising filter (particularly when used to remove / reduce reflections / glare).
• Neutral Density filters are sometimes necessary. For example: when the photographer wants to reduce the amount of light entering the lens to allow a wide apperture and a long exposure (for example, when photographing flowing water).
• UV Filters:
  • Many photographers feel that it is worthwhile to put a UV filter on the front of every lens, on the basis that this will protect the front lens element - from dust, scratches and catastrophic damage if the lens is dropped. In other words, it is a kind of insurance policy against lens damage. I understand that Scott Kelby takes this position.
  • Many (other) photographers believe that this practice is not worthwhile: they may feel that the reduction in image quality (actual or potential) is not justified by this "insurance policy". I have heard Scott Bourne articulate this belief.

I hope this is a fair and sensitive summary ;)