Depends.....
When I first started I used wood props and I hand started the engine it made no odds whatsoever.
When I got an electric starter, the props had a habit of coming loose and it was a hassle unscrewing the front of the (plastic) spinner each time to retighten the prop then get everything lined up again.
When I swtched to APC props, the situation became worse and the cutouts aren't usually big enough on most plastic props anyway, so I in turn had to switch to an alumium spinner. This combination works well when you are using a starter.
So in conclusion, if you hand start then a spinner is just for looks. If you use an electric starter then a spinner is the best surface to use, although you can reverse the rubber to use on the bare shaft.

Recently on a new engine I experimented with a 4 blade prop. The LHS didn't have a 4 blade spinner so I just used the standard nut. Later, I ordered a Harry Higley prop nut. I then found that the starter wouldn't turn the nut so I got a special Sullivan starter cone for prop nuts...

You need something on the front of the prop. Why?
1. AMA rules (ifthey affect you)
2. Spinners make electric starts easier
3. Protects the crankshaft and absorbs some impact when the plane crashes