Yes, we do need both.

Even if you're looking for the Voltaire reason (to paraphrase) "If DAZ|Studio did not exist, it would be necessary for Curious Labs to create one", competition is good.

In any field, when one product has total dominance, everyone else suffers. Back in the bad old days of 96% marketshare of Internet Explorer, Microsoft had near-complete dominance of the web browser market... and Internet Explorer simply languished. Nothing new or exciting came out. Security sucked - any random 13-year-old with a text editor who could type "javascript" or "IFRAME" could destroy your computer.
Then Firefox came along... and suddenly became HUGE (those of us in the Linux community have known about it for years, but it never really took off until there were Windows and OSX versions). Internet Explorer, one of the linchpins to Microsoft's dominance of the web, dropped from 95% marketshare in 2001-2002, down to less than 60% now, and it is still falling hard. Microsoft is still struggling with trying to keep what they do have, even as it copies concepts from everyone else to do it. Meanwhile, this opened the door for Opera to expand, Safari to rise, Chrome to come along, and nowadays a very vibrant market has arisen. End result? We now have a LOT of choices, and new features arriving all the time.

Operating systems? Look at Windows 7 - you would have never seen half the eye-candy and features present in it if it were not for Apple and OSX growing rapidly in strength and marketshare.

Poser? Poser 5 would have languished, and Poser 6/7/Pro/8 would have probably not have half the new features they had if it wasn't for DAZ|Studio's existence and growth. DAZ|Studio would not have had IBL and all its other goodies if it wasn't for Poser's additions of them. The two challenge each other, and we end up the winners.