Stan - My perspective is it depends on the boat/motor and specific issues. I have a buddy with a 640lx with a 90 Merc 2 stroke, 23 p 3 blade SS prop, and that thing has amazing holeshot and runs 50mph GPS, so on that small boat/90merc no need or issues for a foil.
I put one on my PC 175 40 Merc, to do what Al said, help me get on plane quicker when towing a heavy kids/adult in a tube. yup, as Pat said, if I had the 75 Merc or a different prop set up for tubing, i'd probably not need it.
No need on my current Nitro (700lx 115 Carb'd merc), though i'm thinking of a holeshot plate just for grins and giggles, and because I can actualy MAKE it!! LOL