In general I agree with Happy Squirrel's notion that "system sellers" won't turn a console's fortunes around all on their own...usually. But there are some interesting counter-points.
Super NES vs. Genesis: Perhaps too much is made of Donkey Kong Country's importance to Super Nintendo, but it was at least an important factor in turning the tide in Nintendo's favour. I think when you have a close race between two competitors, the "system sellers" suddenly become more important.
Nintendo DS: If you look at the sales, DS in Japan was basically built on system sellers. Third party support was poor, but Nintendo's first party games sold a ton of systems (check out system sales at the launch of Nintendogs, Animal Crossing, Brain Age, and New Super Mario Bros.) and eventually third parties had to support it. In North America DS Lite gets most of the credit but I believe Nintendogs and New Super Mario Bros. deserve some credit as well.
I think with DS the point is that it needed disruptive games that showed what made it good.
Edit - Summary: If everything else is similar, then system-selling games will not allow a console to overcome a huge sales gap against its competition.
