Actually an interesting thread that seems well-balanced. I always love Roger's Lens Rentals articles as they tend to put things in real world perspective.
I have considered adding a full-frame body as well and may do so at some point, but honestly, Canon's recent actions are pushing me toward sticking with APS-C. Why?
Money. It's that simple.
Every time I do the math, reality sinks in and I just can't make the cost-benefit analysis work in favor of full frame.
I'm not earning money from photography. While I would like to explore the possibility in post-retirement, that's a few years down the road and for now, I'm concentrating on improving technique and building up my personal portfolio.
For what it is worth, here is my perspective:
My main interest in full frame is at the wide end. I have been spoiled by the 1.6 magnification for longer lenses and would almost certainly continue to use the 7D for telephoto lenses when I am distance limited. There would be some advantage to being able to use my telephoto lenses with both bodies, but that is a convenience not a necessity.
The cost of entry into full frame will be, at a minimum somewhere around $2,300, and that would be for a refurbished 5DII and 24-105mm lens, the bare minimum needed in my opinion. I already own the 15-85mm EF-S and the Tokina 11-16. So, the wide end is covered with my 7D. The 24-105 would replicate the 15-85 range, but not the 11-16 crop range. So, initially, it would be a compromise that would have to be supplemented by the 7D when ultra-wide is needed.
Now, $2,300 isn't out of the question, but that is for old technology. If I want the 6D and the new 24-70 L lens the cost of entry rises to about $3,500.
But, that is for a single lens kit. To take full advantage of the full-frame I would want to invest in other lenses, such as the 24 2.8 IS prime and probably the 135 f2 prime. After all, what is the point of the full frame if you can't take advantage of its perceived shallower depth of field?
Because I seldom shoot at over ISO 400, I regularly print images at 12x18 with the 7D with no loss of quality. I've gone as high as 18 x 30 without any problem.
Like many photographers today, the vast majority of my pictures end up on the internet at 72dpi. The other use I have for prints is in photo books, which never get larger than 9 x 14 maximum.
So that's where I am at. I can't bring myself to spend well over $2,000 on old technology, I can't justify $3,500 for the minimum of what I would want in new technology and I certainly can't rationalize $4,000 for the 5DIII and kit lens. Plus, I am not anxious to start adding new lenses at $800-$1,200 a pop.
So, this is where Canon has put me: I am happy with my 7D. I consider it to still be the absolutely best APS-C camera on the market today even three years after introduction. So, I will wait and see what Canon does next spring. I expect to see a 7DII and I expect it will have some marginal improvements. Even if it is released at the same price as the 6D, I think it will be a better camera and I won't have to buy wider lenses. Even if I decide to add the 17-55 2.8 lens (refurbished) it will still be significantly less costly than the 6D and 24-70 f4 and a full stop faster. I firmly believe that there will be a 7DII, for reasons that have been well-documented on this forum. But, even if it doesn't materialize, I won't have lost anything and I can decide what to do when that becomes known.
I am happy with my current kit and frankly have more than I can reasonably carry in a bag anyway. Adding lenses is always fun and tempting, but honestly, I don't need anything right now.
My point: Every case is different. But, I would be a prime candidate for the jump to full-frame. But Canon has raised the cost of entry so significantly that when I compare the cost, versus the benefits I cannot justify it. I believe I will remain an APS-C shooter for quite some time.