My input on your post, offset cannot be adjusted or more like it cannot be adjust without affecting the hosel integrity. Thus globally majority of clubmakers, even the rep's in the tour vans don't bother.
As for how it can affect the swing, imagine this, you are always swinging with an offset club and are able to to hit it straight, as mentioned by some of the brothers above, the offset helps to square the club face off. which means the swing taken/used would be a slice biased swing.
if using the same swing with a minimal/ no offset club, the likely direction of the ball would be right.
Whether or not they are beneficial really depends on what is the players objective.