Some people find it's easier to not have to get a kid in/out of the 3rd row. Particularly if there are car seats attached to the 2nd row. Or, they might also be a bit concerned with having less crumple space in the 3rd row.
And yes, I have every reason in the world to buy a Toyota. Two bro-in-laws are Toyota mechanics. One is the shop foreman in one of the biggest dealers in my state. The other one is more neutral, but even the die hard Toyota apologist one admits the new 2011 Sienna is not quite the top choice anymore (he owns a 2006).
So yes, if you're considering 8 seat minivans, the 2011 Odyssey is CLEARLY superior. The changes to the 2011 Sienna are frankly head scratchingly stupid. It's something you'd expect the domestics to have done back in the 90s. Also, being from Los Angeles, the interior is vital to me since I'm used to spending so much time in there. The new Sienna feels horrifically cheap. Frankly, a lot of the new Toyotas are heading in that direction.
However, if you're going used, the 2005-2010 Sienna 8 seater model was clearly the best 8 seater minivan. The 7 seater was good too, but since the Honda drove better, I give the edge to the Odyssey if you're looking at 7 seaters. For 8 seats tho, the 8 seat Ody is nowhere near as user friendly as the 8 seater Sienna.
But for 2011, it's like Honda looked at how the 2005-2010 Sienna was better, and improved on that. Toyota apparently looked at an old GM minivan and see what they could do to make the car crappy enough so sales drop through the basement.