Not having them is worse than having them.See, you group them together and say "HD consoles." Those third party mutliplats may convince someone to buy a console but they won't help anyone decide on your console.
This isn't a difficult concept to grasp. The titles that are making waves in the industry, or that most gamers pay attention to, are assets to have in your portfolio. Regardless of whether they're available on other platforms or not, they can sway a person from not choosing your console if you don't have them. It's better to have them and then work on your own exclusive games, services and brand to entice people to choose you.
Few people are going to choose a console that has Mario and Zelda over a console that has Call of Duty and Grand Theft Auto. The latter two are much more significant today. And it's not just those titles. It's dozens of other third party titles, all ranging from decent sellers to blockbusters that will not be found on a Nintendo console.
All of that has an effect. It builds up.
Leaving behind where most of the industry is heading to go throw pebbles in that pond over there in the corner just means you'll be increasingly isolated.