Purina is one of the companies responsible for nutrition research and their foods are good and adequate. Some reasons your vet would recommend it are
1) It is way better than than the cheap junk many kittens are fed
IMO, Purina is part of the "cheap junk" listings of cat food. corn and wheat gluten have no place in a carnivore's diet
2) If it's on your vet's shelf, your vet gets a cut of the price
3) As it is your first kitten maybe you would not know where to start.
4) Research into grain-free diets, raw diets, herbal/veggie/fruit supplements is minimal, and your vet may be one who doesn't believe anything until it is proven in research.
Here is the proof. Cats have a very short digestive tract that can't extract protein from vegitation efficiently, it is made for animal protein. Cat's lack the enzyme, amylase that break down starches into carbs for energy, cats get their energy from meat protein and fat. Cats teeth are not made for breaking down plant cellular matter, but for ripping flesh and chewing bones. Cats need taurine, only found in it's natural state in uncooked meat/organs. I could go on, but I am sure you get the message. If I can figure this out, so should a vet.
5) There are, in fact, so many brands and varieties out there that your vet hasn't the time to become familiar with them.
That's a vet's job if he/she is expected to give nutritional advice, that vet should know what is good and not good foods for a cat.
Companies like Petcurean use the research Purina has done to make a food that has all the nutrients your kitten does, and then add in their own features: grain-free, low processing, no artificial preservatives, various holistic herb/fruit/veggie supplements... If you look at the ingredients label on your bag of Now!, you will see a long list of vitamins, minerals, amino acids, etc. that they have added to ensure your kitty gets what she needs. You will also an AAFCO statement, don't feed anything without one on the label.
I make my own homemade raw, no stamp on it. I can tell you had I not changed my Puddles's food a few years ago from cheap kibble (probably made by Purina), to a quality canned and raw diet, she would probably be dead by now. When I wasn't making my own raw, I was feeding a commercial made raw and the vets (4 different ones in total), could not believe the muscle mass all my fosters had and how healthy they were.
I do believe that you have made a good choice for your kitten. But keep in mind, every kitten is an individual, if your kitten is healthy, the food is working.
All nutritionally complete food will show that your kitten is healthy, so do young smokers look healthy. It takes years to see the toll of what an unhealthy lifestyle will do.
However, if I were you, I would seriously consider a new vet. As a person who is interested in newer ideas, and a more holistic approach to your kitten's health and nutrition, you would be better matched with a vet who is comfortable with your philosophies. I would also have some concerns that a product-pushing vet might also encourage you to do unnecessary tests or procedures, rather than doing what's best for your kitten and listening to your choices and opinions.