i dont think its necessary to go FF. Personally i just feel its just an ego... FF also got to depends on what model you are getting. Always choose 1ds mark II or 1ds mark III
If getting old FF, New crop frame will be much better interms of IQ, Speed, Technology, LCD and more...
As for the big sensor, The difference is not huge also... Most dslrs have noise at the higest ISOs setting which make a comparision very mild difference.
For Eg, 5D is pro uncle camera already, but if you compare to new semi-pro 50D, its totally losing out. Same as those older FF camera.
If i wanna get another FF, I will prob get a 1ds mark III, and the least is IDs mark II. I wouldnt even consider any other FF older than 1ds mark II.
Dust problem is a real pain, obsolete parts & technology is another problem, not to mention also the high cost for average income pple... Ds II, Ds III appro $3k-$8K