I thought people buy full frame mainly for the shallow depth of field, cheap wide angle lens, as that may be major things APSC cant give.
but NEX full frame may be cheaper than the Pentax, Canon, or Nikon's full frame, as it targets more for consumers.
by the way, NEX system also can use those range finder lens and those ancient under-used FD, MD manual lens without losing optical performance, provided that you are willing to give up the fast AF, A mode, and rest.
but you get one full frame camera with freedom to use any good, cheap, modern, ancient, dusty lens. and you still have choice to use fast mordern AF lens with everything automatic and fast. (provided it is relative cheap, half price of what Canon or Nikon charges.)
FF is mainly for less DOF (but it cuts both ways) and the larger viewfinder.
The wide angle part is an old half truth, passed on till today as 'truth'. At the beginning of the switch to digital, there was only crop sensors, so the widest lenses on 35mm format became less wide. Later designed for crop sensor UWA were designed, eliminating this problem.
The widest on crop is 12mm (ie. Sigma 8-16mm). The widest on 35mm format is also 12mm (eg. Nikkor 12-24).
As Detrius points out. There is no upcoming FF NEX. There is a rumored FF SLT. Though I won't be surprised if FF NEX becomes a reality within the next 2 yrs.
But then a FF sensor is not going to be cheap. The yield loss on a FF wafer will be significantly higher than crop. That has been the unchanged reality, though overall costs have come down over the years.
A NEX is not really a cheaper camera if you look at it closely. Less buttons; No SR; no viewfinder. The D5100 uses the same sensor with a better interface and has a viewfinder (ie. more mechanicals) at about the same cost.
So sensor price will be a large factor affecting cost of a camera.
Of course NEX can use many lens types. That is its attraction, esp. to the RF users, whose lenses are manual anyway, are small to make use of a smaller body like the NEX and have a next better option in a $9000 Leica M9.
BTW, many of the good RF lenses are well sought after and not cheap at all.
In the end, say a FF NEX cost in the region of $2.5K-$3K. Is it worth it as a MF lens platform?
Only each individual can answer that.
If the individual has plenty of MF lenses, then it can be a viable option if he/she does not need AF speed at all.
If the person has mostly AF lenses, then to me that is too high a cost.
I'd wait for it to appear 2nd hand on BnS at $1.5k or lower.
Also, Sony price strategy is there for everyone to see.
Cheap ones are all so=so commercial lenses. The good ones in that category are the Minolta ones (rebadged as Sony).
You want better lenses, you pay big money for the CZ ones.
So no free cheap/good/AF from Sonny boy