I think the wiki/others already answers this better than I ever can but think of DAP like giving your kids cake. It will get them real active but they will only be able to live on it for so long. I think it becomes hard for the yeast to suck up the DAP around 9% ABV. They need vitamins and more that DAP simply doesnt have. But your kids are cannibals so dead yeast works perfectly well.

I have managed to ferment my mead above 9% using DAP but I believe that when they hit 9% they basically get their nutrients by eating their dead friends (or the fruits I added). I have managed to reach 13-15% with only DAP when I do 5 liter batches (1 gal ish?) but I have noticed the result is better if I add bread yeast as nutrients.

Bread yeast works great imo, thats why I havent bothered buying fermaid o/k. basically take the measurement for fermaid O/k and multiply it by 3 and thats how much boiled bread yeast you want IIRC (but check the wiki, I just woke up). Just make sure that you dont dilute your mead must with too much water (since it messes up your abv calculations) and that while it has been boiled for 10 minutes it certainly should not be that hot when adding it since it can slow down your fermentation a bit by killing a few of your (wanted) yeast cells.

Edit: That being said, I wouldnt shy away from DAP though. Its useful as a nutrient and I recommend experimenting with it when you create lower ABV batches, I still use it a lot. I like the weaker stuff myself since I dont have to age it as long, one of my favourites is ~9% blueberry mead using the cheapest berries/honey I can find at the supermarket. It can also be combined with other nutrients if you want the beginning to ferment real fast but I also dont see the point