Would you care to provide a real world example where a major 6 chord functions as a V? That would really help to understand your position.
Your question drove me through the Realbook, and sweating heavily, phew..
I started looking up the obvious examples: There will never be another you, I can't give you anything but love, Darn that dream, Paper Doll...
- all of them with melodies ending from maj. 3rd (the 6th to V) to key - only to find out that all of those composers wrote a dom7th chord under that note like if their noses were bleeding... a good lesson in humility. I really would like to know why they did that, writing 6th against 7th just like that. Probably Ed Fuqua knows this.
Far more common, (especially around the 1940's) - as Ed explained - the 6th is used to color certain chords. The 6th is not the melody note, and certainly not the intended bass note (like b5 or b9 sometimes are), it's just there to add some varnish. In many cases, you''ll find this feature in song endings -they are all over the RealBook. The harmonic function there is - I -, of course.
Nice case of 6th-chord as subdominant is the famous plagal ending of My Way (F6 to C), where the 6th is the melody note.
And now for the dominant 6th... (drum roll, please). They were not as common as I thought they would be. But two clearcut examples from the RealBook where a 6th chord is dominant are Hot Toddy and Wives and Lovers.
Anyway, all of this only underlines the importance of being aware of harmonic functions, wether we play them or write them.