I'd keep the 3rd round as it is, but there's absolutely no reason for the 4th round onwards to take up weekends.
Yeah, agreed actually. For cultural/traditional reasons I can see why the 3rd round should stay as is. In many ways that's the high point of the competition, and there's a buzz around because of the turn of the year and this that and the other. But like you say, from then on, it becomes fairly ergh.
My football watching attitude tends to change quite heavily depending on whether it's a weekday or a weekend. During the week I am much more open to watching anything, especially in winter, when it's dark, there's not a lot else to do, and you come home from work just looking for something to vaguely entertain you for a few hours. Football fits the bill well, as even if a game is awful I won't mind, because I'd probably have only spend that time messing around online or watching Pawn Stars or some other crap.
At the weekend, however, my attitude changes. I like to actually spend that time doing something worthwhile. So, in other words, if I'm to set aside time to watch football I'm expecting it to be worth me doing so. When it's a big Prem game that's fine, and I will do so, but when it's the FA Cup I just cbf. It's just not enough of a hook for me at all.
Another thing I hate is the FA Cup final kicking off at 5pm or whatever it is these days. Just terrible. Mind you, I still wish the Champions League final was held on a Wednesday like it used to be, so what do I know.