Hmmm now if ......................... and I say IF your scope is set up right so that when you raise your rifle to your shoulder so that the sight aligns with your eye without you having to move your head to find it :roll: and you have a reasonable powered scope. No more than 6x mag lower is better and 4x is good one. Then shooting moving ............................. fairly slowly moving targets is no problem. Here in the UK the shooting of moving game is frowned upon unless it's wounded then it's seen as humane.
No I didn't make that up either.
In low light a scope should actually help by increasing the light reaching the eye providing of course the reticle is heavy enough to be seen in the gloom :wink: .
Now back to the Iron sight issue, this is one which I have been looking at and want to get more use out of some classic hunting rifles which do have Iron sights. To get more used to and better at shooting with them I am currently practising with a BSA Sportsman Five .22 R/F rifle which has a P-H 16B sporting peep sight fitted combined with the standard BSA blade fore sight. Waiting in the wings I have a P-H 6E aperture sporting sight on a BSA Majestic Feather Weight in .270 and a BSA Model D with a P-H 6EH in .303. Once I get more used to shooting with these and my groups tighten up more then I will break out some express sighted rifles and give them a whirl. My goal is to be able to consitantly group to under 2" at 100 yards from various field shooting positions like sitting and standing using a single stick and then free hand standing at 75 meters. Once I can do that consitantly then I will judge that I am good to go in the field :-) .
Now I did speak to my friend the gamekeep who I was hunting Roe Buck with last weekend and we discussed my using the sporting aperture sight which out to 75 meters I can group inside my own set limits from a steady shooting position. However he pointed out that due to the crops being so high, the Rape Seed is chest high, that it would severly limit my chances of a seeing the Deer, here is an example of what I mean:-

This is a nice 4 point Roe Buck resting up, a smaller Buck was laying up about 10 feet from him and all you could see was an ear :shock: and this was from the vantage point of an elevated box stand. From the ground they were not visible at all and we did not spot them until climbing the ladder to the stand where the plan was to wait a while and see what came out. The weather was against us there being a high wind and this little corner was sheltered. Whilst climbing the ladder we saw 3 Roe Deer that were not visable for the ground due to the height of the grass.
Once the Buck stood up to clean and browse then yes I could have used the iron sights and he was only about 75 yards away and actually came to about 35 yards away before entering the wood behind us. I did not shoot him except with the camera as I was looking to get a 6 point Buck. This is what the cover was like on most of the wooded areas. Some plantations and natural copses were heavier in cover :shock: in a few weeks the Braken will grow up and open out then you can stand 10 feet froma Fallow Der and not see them :oops: :-

Not so good for iron sights a scope helps here and using just irons limits your chances just as Cliff said it would, once the Deer come into clearings or onto the fields to browse then you have a good chance with the iron sights. I will continue to practice with them with the goal of hunting with them again. :-)
More photos of the two Bucks are in the Deer Hunting forum if anyone wishes to see them.