Full frame a necessity??

Is a full frame camera necesary for everyone or more so just for certain applications?

Personally, I think FF's are best for landscape, other wide angle and some portrait work. For wildlife, you lose the "zoom bonus" of the crop sensor. I would love to have a FF but, for me, the cost does not justify it.

I've just been trying to the learn the difference between full frame and crop sensor. Wondering if it's worth to buy a full frame.

It's important if you want to shoot very low light and need low noise, or if you want a really high pixel count. If you're asking that question, you probably don't need to worry too much about the difference between the two unless you have a bunch of money and don't want to have to worry about buying another camera in a few years.

Cameras have always come in a variety of formats (the size of the film frame or sensor). The various formats have pros and cons. Each photographer has to weigh those, and decide what the best choice is for them. Consider two of the 20th century's most famous photographers: Ansel Adams chose 8x10 format and Henri Cartier-Bresson chose 35mm. They chose the tools that were best suited for the subjects they shot and their personal style. If you don't know why you need to go to full frame then stick with what you have.

The smaller the format the greater the ratio of enlargement required to get to a particular print size. For instance a 35mm neg has to be enlarged many more times to get to 8"x10" print than a 4"x5" neg. The greater the enlargement ratio the more small flaws are revealed. It doesn't quite work the same way with digital, but you can sort of think of it like that. A lot of folks think bigger is better in all manner of things. On the other hand modern 35mm film can produce amazing image quality, and in my opinion an 8mp APS-C DSLR made by any of the major manufacturers can match or beat that quality level in the right hands.

I'm shooting with a 5DII for it's low noise high ISO quality (for weddings and live music). Actually when pixel peeping it appears to be more noisy than my older 5D, but when prints are made the extra resolution does reduce the noise. My walls are covered in landscape photographs ranging from 8"x12" to 20"x30" print size shot with several Canon DSLRs: 20D ( 8mp APS-C), 40D (10mp APS-C), 5D (12mp 35mm), and 5DII (21mp 35mm). They all look good, IMHO, and experienced photographers can't seem to pick which photo is from which camera with much accuracy. For me the secret to excellent sharpness in big prints is to use a tripod, MLU, and cable release.

Why would anyone want to carry a heavier camera?

What sort of photography are you into?

I did notice you have a Canon XSi which Has been my mainstay for the past few years - backed up by an XT.
Check my galleries and see if you can find many cases where a higher-pixel count, heavier, full frame would have helped. I think the count will be low so I am sticking with what I have for now. II did get a Zeikos battery grip for the XSi for those times when I want to shoot for days without recharge due to the extra battery (and its handy for vertical shots).
Sometimes I think it looks "funny" when my 50mm prime is mounted - all camera and itty bitty lens. Just kidding but it does look different.

Well I would say that going FF is needed if you shoot mostly scapes, shoot at night alot, or in the studio. I shoot wildlife and landscapes so yeah I could use a good FF but, since I do shoot wildlife I will stick withthe cropped senesor. I have the Nikon D300 and a 10.5 fisheye so I have the wide covered for DX. Full frame is better for printing but for the normal consumer and the size they print at going FF is of no benefit.

Full frame allows you to use the lenses's at their native focal length and distortion. Allows more DOF as well. Honestly Full Frame is the best at everything besides birding and some sports. That's about it. (in the 35mm format).

I do miss reach time to time though.

It is not necessary for taking good pictures though.

yeh, anything involving more reach then a crop camera is gonna be a bit better.

Everything else, a FF camera will have more details, shallower DOF and will use more of the lens (as they're built to be used).