Tommy McClain / AzBat did a great job of answering why backwards-compatibility is important and why there is more than a "little incentive" to do it.
So I'll just respond to this:
And they also had a good undertanding of what BC is really worth to a platform and the did the math (same as Sony, this gen and next) and came to the conclusion that emulation is a hell of a lot of cost for very little gain.
This is where I don't agree. Sony, Microsoft, you or I or anyone else in this thread does not know yet what the gain is or will be. We might never know, because the decisions have been made and certain things can't be changed anymore.
I also disagree that we can use past generations as a pointer to know what works and what doesn't this coming generation. Even if Sony or Microsoft had means to check how often users played old generation games on their new console (lets say they have some kind of background logger), then they will only know how much it was used on current generation consoles with games from last.
Now, as I have explained numerous times already: Talking strictly as a PS gamer (but it applies too to Xbox gamers), there might have been less incentive to play PS2 games on PS3 hardware. Why? Going into this generation, we saw a shift. While PS2 and older consoles were predominently played on CRT televisions at 480/576i resolutions, we came into this generation with HD-ready and FullHD sets. If you've ever set up an old generation game on current consoles on modern LCD and Plasma television sets, you'll see how painfull most games are. Some games have aged well - but for the most part, especially PS2 games have not. Aliasing is horrendous, bland textures, texture shimmering... and if you move on to even older titles, you'll even see clipping and a mess of pixels at an even lower resolution.
This generation not only introduced online-gaming on a mass-market scale, but it also introduced a high standard of resolution (sub-HD to Full-HD games) to match our television sets and much higher image-quality.
Talking strictly about resolution and image-quality - going into next generation, we won't see a big a leap as we did before. The games will still run at the same resolution for the most part, image-quality might be improved, but not to the levels we went coming from PS2 games into this generation. In other words, the next generation games will not look leaps and bounds better than what we have now - and everything we've seen at E3 so far underlines this. Sure, newer games are more complex, more stuff going on, better animations, more detail etc - but on a level of image-quality and resolution, they are not. So in other words, playing PS3/X360 games on newer generation hardware would not be as painfull to the eyes as it has been a generation before.
Then, there's the point of online-play. Last generation we did not have mass-market online play. To most people, this was new coming into this generation. Before that, we didn't play games socially over the internet (not on consoles anyway), so when coming to this generation of PS3 and X360, we didn't connect with our friends and buddies.
Fast forward to today and there's a large majority of PS3 and X360 gamers who play games socially online. We have extensive friends list, a large catalog of online and offline games, both purchased online and offline. So when the point comes where we do upgrade to new consoles, the friends who we play and socialize with online hold a bigger influence over our gaming habbits than before. If for instance, a large majority of gamers still play games online with their buddies, they might hold off with buying the new consoles because it would mean they could not continue to play together.
The other thing is, as AzBat pointed out, that the bigger the investment you've made on this generation, the more you are losing. While old-games are effectively being played less at some point on newer hardware, supplying backwards-compatibility gives us an incentive to stick with our platform. It adds to the value of the brand. If you don't have it, there's less reason to stick with your platform and with the way people and buddies are connected in this new online world, people jumping ship could have a bigger effect than in the past. So if going onto the next platform, suddenly all your friends and buddies are "offline", it's probably easy to explain that they must of jumped ship. Or if you want to continue playing with your friends - it's easier to get what they are getting because games are now connecting people (which was not the case last generation).
I have already had numerous friends and online buddies that I have played with online ask me what I thought about Xbox One or PS4 and which I would be getting. I'm not sure how many I have influenced by saying what I'll be getting, but playing together will be just as if not even a bigger factor next generation. Our decisions influence each other and every factor to bind customers to a platform is important.
If Android or iOS apps weren't compatible across phones running the same OS, people would be perhaps switching phones more. If I ever wanted to switch to an iPhone, I would immediately lose all my apps that I bought. However, knowing that I will still be able to use them on my next phone Android helps me stick to what I know, even if perhaps the next iPhone would be a great phone.
In the end, we on these technical forums know that there is a technical limitation to backwards-compatibility. We know it's not easy, that it comes at a (significant) cost. Yet, I am willing to argue that the value is higher than some here argue. Even if consumers will not use it in the grand scheme of things, consumer psychology is a factor not to be underestimated. And going into next generation; your current loyal userbase is the one you at least want to sell to and not start from scratch.