Data sources, what's the big deal?

Like most begining family historians I didn't ask for this role, nor have I had the benefit of some apprenticeship of any sort.  As I began reading the beginners guides and different sources were discussed I got the impression that a photostatic copy of a document was on equal footing as the original.  a recent discussion made me uestion if that assertion was accurate.  Is a photocopy or digital scan of a document my dad has in his possesion just as good as me having the document in my possession?

Processing and manipulation of the image is a double edged sword.  Done professionally and done properly and documenting each step, is a measure of conservancy that can restore a faded or damaged document to it's pristine glory. Or it could be a way of forging a document.  As we get further into our research it just won't be possible for us to handle every piece of original data on every ancestor.  So what are your thoughts on this topic?