As to getting dirty, a finish doesn't prevent that, and sometimes just makes it worse as the finish deteriorates. Alan Peters in his book, Cabinet Making: A Professional Approach Says that unfinished is his favorite "finish" for kitchen tables. This approach has been used for decades by his mentors and himself, and for centuries by craftsmen before them... I have comparatively little experience with it, but in a way it makes sense.
As the table gets dirty, you take some hot soapy water and scrub it down. The water will tend to swell up and fill in dents. Scrubbing keeps the dirty nastiness from building up and he says that over a period of decades, a nice patina develops.
"History is strewn with the wrecks of nations which have gained a little progressiveness at the cost of a great deal of hard manliness, and have thus prepared themselves for destruction as soon as the movements of the world gave a chance for it." -Walter Bagehot