In every market that exists the same rule aplies:
More competition = Better deal for consumers
That is of course excluding the gaming industry...
What we have now is what I believe to be a perfect balance, 3 consoles offering consumers a choice, preventing a monopoly and each with its own set of great games. What is also good about our current situation is that one is completely different to the other two (the wii) and sets itself apart. The 360 and PS3 have differences as well but they are not as set apart as the wii is.
When you think about the annoyance over shoddy ports from either console which causes grief to the consumer, and the annoyance of not being able to play the game you want because its on another console you can see that more competition would definetely be a bad thing.
The console market seems balanced at a equilibriam at the moment: providing a fair amount of choice for consumers whilst preventing huge game divisions between many consoles.
The next gen I fear could change this balance or if not the next gen, the one after that. This is due to the threat of companies like Apple joining the console market. Imagine having about 5 similar machines to choose from. As a gamer one is likely to desire exclusive games on each platform and if he / she does choose one is more than likely to end up playing a few shoddy ports.
So the console market is saturated enough!
DONT RUIN WHAT WE HAVE!