Generally having multiple consoles on the market tends to force devs to stay competitive. The issue is also pricing. If you have no competitors then you control pricing. With no competing product on the market then prices will skyrocket because hardware makers can do so without any repercussion.
A competitive market keeps things fair and balanced. While it would be nice for console hardware makers to adopt a unified programming language for easy porting across multiple hardware, but the reality of this situation is far more dire then hopeful.
Having variety means more choice. We do not need developers to become complacent and resting on their laurels when one machine rules them all. Of course there may be some merits to having a single console, but thinking about big picture the negatives far out weight the positives.