I would say there are a few reasons. First and foremost is that it's not a "natural" product so people who prefer to avoid chemicals wouldn't want to add it. Second, if you notice the amount used in the recipe is teeny-tiny, in this case 1 tsp to nearly 6 lbs of mix so you can easily go overboard and your sausage will be pretty close in texture to a tennis ball.
I get mine from Butcher-Packer (
http://www.butcher-packer.com/index.php?main_page=product_info&cPath=26_103&products_id=894) and used the prescibed amount of 0.5% [2 oz for 25 lbs meat] for some bulk italian sausage. I mixed the sausage and put it into ziploc bags and froze the meat flat for storage. When I fried up one of the large ziploc size patties I planned on just breaking it up into pieces to add in some sauce but couldn't hardly cut through it with a wooden spoon. Decided to wait until fully cooked through and had to use a knife to dice it and it was still like cutting through hard meaty-jello. Definately not good eats....
After a little experimentation I found that I prefer to use about .17%-ish based on the weight of the meat and typically skip it if using NFDM. According to the book Modernist Cuisine the legal limit is 0.5% but percentages higher than 0.3% can leave an off-taste.