My DPMS LR-308 is set up that way.
Basically it goes like this:
- I like the most rigidly attached iron sights that I can get. If there'd been an AR10A2 available, I would have gotten one. So, I settled for having only the front sight permanently mounted.
- I do have a detachable carry handle, and it works pretty well. The fact that I can switch between the carry handle and a scope is a bonus.
- Until you get down to maybe 2.5x, you don't see the front sight base through the scope (even with a low mounted scope). It looks weird from the side, but it works when you've got it at your shoulder.
The way that I had it set up originally was with a folding rear sight and a quick-detachable scope mount. I primarily used the scope, but if it ever got damaged (that actually happened once) or if I needed to rule out the scope as a factor in some accuracy related issue (that's happened too), I could remove it and flip up the rear iron sight and keep shooting. I left it zeroed for 200 yards with the load I used most, and it worked.
Basically, having the railed flat-top receiver added some versatility to the rifle, and it's served me well. Now, I've gone back to having the carry handle mounted - so it would be legal for use as a Service Rifle in NRA (not CMP) High-Power. If I wanted a red-dot (with or without a magnifier) on it , I could co-witness it with the front sight and either a fixed or folding rear-sight if I wanted. It wouldn't make a lot of sense on a 20" heavy barrel/A2 stock, but I *could* do it - maybe for a three-gun competition or something.
The bottom line as I see it is three-fold. It's more stable than a folding (or fixed detachable type) front sight. It's always up and ready to use. It doesn't get in the way near as much as you'd think.