I don't see the point. For the price that most are asking you can buy an M1A(M14) for the same price and its more reliable.

I also hear that it eliminates the use of the foward assist. Is this true?

Repeatability.  Once you float an AR-10, you don't have to mess with it.  Even with a unitized gas system, the M-1A/M-14 will eventually lose its accuracy, and need to be re-bedded.

Also, parts commonality and personal preference.  The M-1A/M-14 system is definitely for right handers, even though I have one and shoot lefty.  A pistol grip does have an effect on being able to get a good stock weld.

Some people just like them better.