Note 2: If you think the poll is biased or incomplete please make your own, better poll.
Note 3: I'm aware this issue has been heavily discussed, so please avoid "not this again" kind of spam posts, please.
So far the possible explanations for the merging of cuirass and greaves seem to be:
1. Increasing the number of NPC's on screen: As Matt said in the Q&A: "it renders a lot faster too, so we can put more people on screen". This might be true, but the magnitude isn't clear. I find it hard to believe the removal of one single armor slot frees enough memory to render aditional NPC's, but this is just common sense, I have no proof, of course.
2. Simplifying the game for console players: Let's take a look at some famous console games:
Forza Motorsport 4: 500 cars that can be modified, edited and tweaked to meet your taste and game style.
Pokemon Black/White: 156 new pokemon, 649 in total, with unique abilities, weakness and attacks, and complex breeding.
Fifa 11: 80 leagues, 500 teams with unique kits, 12.500 players, different boot types, body builds, complex player creation system.
The more features a game has, the better. Ask any MW2 devoted fan (65 weapons and 16 attachments) if they would like to have more weapons or attachments or less.
Please stop using this argument, its quite insulting, and makes no sense at all.
3. Requirements of the new Skyrim visual style. Almost all of the screenshots we've seen so far show greave-less characters, and, as Matt said on the Q&A: "This helps create armor styles that have the look we needed for Skyrim. In most of the Nordic designs we created, the upper armor would completely cover the lower armor, making it unnecessary. We get much better visual results combining those pieces"
This makes perfect sense to me, but it doesn't justify the removal of the greaves slot at all. Some armour, even most of it,could use both slots (like robes and those shown on http://www.zonablade.com/wp-content/uploads/2011/03/Skyrim-Castellano.jpg), but this isn't incompatible with some one-slot traditional cuirass or greaves.
4. More armor variation: Again, quoting Matt in the Q&A: "the number and variation types are more than we’ve ever had". This seems quite absurd to me, because having separate slots for cuirass and greaves allows endless possible combinations. I understand many of you consider mix-and-matching armor as "lame", and I see your point, but, as I said before, full body armor isn't incompatible with some one-slot traditional armor. I find this reason a poor justification, poor indeed.
5. Requirements of the new Skyrim smithing system: Again, quoting Matt in the Q&A: "The forge is actually used to create new weapons and armor from raw materials". Hopefully this new system not only gives us a reason to collect those wolf/mammoth pelts and different ore types, but also allows endless customization options by combining pauldrons, skirts, chest pieces, greaves, belts, pelts...
If that is the case... merging cuirass and greaves is great news! why? because we get to customize the whole armor, without limits like clipping issues/size of greaves/type of greaves, allowing us to create truly unique tailor made armor.
I know Matt didn't justify the removal of one armor slot because of this, but it seems to me as the only plausible reason to make such a bold decision besides from option 6... So I'm going to be hopefull and believe this is the correct reason... at least until we know how the forge works.
6. Lazyness / Not a priority. Some of you migh argue that Bethesda is making a huge sandbox quest filled game, and this might mean some details, such as armor slots, aren't a priority. Of course, one-slot armor is easier to handle and design than two-slot armor (clipping, compatibility, design issues...) so this could be the real reason behind the decision too.
Anyway, this is my personal opinion, based on common sense and the little info I'm aware of. Feel free to bash it... with arguments please.