One obvious advantage of a larger sensor is that you can have larger pixels in the sensor. All things being equal this should give a higher quality image. However a smaller pixel made two years from now with improved technology may give a better quality image than a pixel twice as large made with today's technology.
Full frame is only of value by itself if you have a collection of full frame lenses already and want to keep the same perspective as you did when you shot those lenses with a 35 mm SLR. Such as a portrait photographer who wants to shoot faces with an 80 mm lens.
1.5x dSLRs have a lot of advantages. They can be made smaller and lighter and presumably cheaper. Same for the DX lenses with the additional optical advantages mentioned above as they are made for digital sensors.
If you have a large megapixel full frame then you have to have a lot of memory and computing power to process that image (in the camera and out of it). You will also need a huge memory card. What do those 8 GB CF cards go for? I used a 512 card with my 5000 and it worked great. Now I have to use a 2 GB card with my D100 and carry an extra 40 GB HD to store the images.
Rumor has it that some future Nikons may use a 1.4x or 1.3x sensor and that supposedly the DX lenses will work fine on them. Supposedly without vignetting.
As mentioned above the DX lenses work on full frame. Not just to their widest zoom. But the focal length they work to is very similar to the widest focal length that their full fame comparable lens zooms to. Meaning the 12-24 has the same angle of coverage as the 17-35 when they both are at their widest practical zoom range.
If you really think you need a full frame dSLR then how would you be satisfied with anything less than a Hasselbald H1 with it's 20,000$ digital back? That would make a full frame camera seem like a joke. And why would anyone ever have taken down a Nikonos instead of a Hasse in a Gates housing? After all they use to advertise they would give you a free South Seas dive vacation every time you bought one. Those medium formate negatives were clearly superior to those from a 35 mm camera.
Of course a 1000$ full frame camera would be a different story. But you would still have to deal with the large files.
I have not heard of those photographers using full frame cameras complaining about the supposed optical failings of 35 mm lenses on digital sensors as claimed in the Olympus study. That study is intended to justify the 4:3 format system. I should state that I've been told that in their digital studios they find the quality of the Oly dSLR to be as good as the 6 MP cameras by the other manufactures.