Is a second floor that important??

To make them move around a bit!! If, like me, you have restricted space and can only manage a 2x3 or 2x4, then a second full level is good to give a little extra space. A loft is all that's necessary if your bottom level meets the recommended sizes. You could put hay and pellets in the loft and they then have to exercise to get to it.

Also, some pigs love being higher up. I know my pigs adore being able to kiss my nose through the bars while I sit next to the cage.

Well, we have just a small 1x3... for no other reason than to let our piggies climb up to for a better view of what's going on in the house.

I have a swing that's suspended from the second level (it looks like a tire swing w/o the hole in the middle) and my guys love to jump on the swing - they seem to like the movement of the swing - they also have a hammock suspended from part of the second level - I wish I could do pictures but I don't have a digital camera - but I am convinced - different levels in the cage "inspires" piggies to move and jump about more especially if you put hay and veggies in strategic places.

Lofts and upper levels do encourage more exercise and exploration. It's really personal choice though whether you want to build them.

I build levels onto a cage because it gives them a place to go to get away from each other. For Cinnamon and Orbit the 1x2 loft keeps them from getting in serious fights. When Cinnamon gets in one of his rumbling moods Orbit retreats to the loft and stands on the top of the ramp facing off against Cinnamon. Without the loft Orbit got chased all over the place and stressed out. All he did was hide in the corner. Now he has somewhere to go that he can easily protect from Cinnamon. Before I put that in Orbit ended up losing an eye and had a shoulder wound down to the muscle. Since then the worst that's happened is missing fur and Cinnamon got a scrape on his cheek from Orbit's teeth. I'm sure he deserved it.

I built my upper level for the same reason as everyone else-to give them a little obstacle and to promote excersise.

I initially just built mine because everyone else seemed to have done it, but now I see that they love sleeping and snuggling up there, possibly because they get to look down on the world! It's also a good place to go if they're not friends as aqh88 says.