I normally use 15lb test on a baitcaster with heavier heads and have great success

 

what is the reasons/benefits for using a spinning rod and lighter line for this method?  

The larger diameter line will interfere with the natural action of the lighter head- My rule of thumb is anything over 1/4 oz with a larger bait- -Baitcaster- 1/4 oz or lighter with a smaller bait like worm- -spinning rod w 6-8 lb test.

I normally use the heavier ones.  Does the finesse method result in greater numbers?  You never hear a pro mention heavier shaky heads, always finesse.  

I normally use the heavier ones. Does the finesse method result in greater numbers? You never hear a pro mention heavier shaky heads, always finesse.

Typically what I have seen is that if a pro has to pull out a worm, then it is finesse time because larger baits aren't working too well.

Doesn't necessarily mean greater numbers.