Do you need to scan with a HUD - YES!
Ok the information is "there" but part of scanning is a controlled focussing of attention. Just staring at the wiggly lines means nothing, however close together the information is.
Why can't I practice IMC or night raw data? If it is dangerous to practice, then it is dangerous to do it for real should the need arise. The need won't arise on a nice sunny day. Flying in cloud is different, even if you look down, the aircraft response to the bumps for a start.
On the Airbus, things are a little different, than the 757 or 767. We do not have a trend vector or a speed vector. In an Airbus, the required level of manual reversion is different to that of a different type. I have no need to practice DECCA or LORAN as I have no possible way of ever needing them in the aircraft I fly. There is a need to fly without the FD or the AP though, so let me practice.
The human is extremely good at doing complex tasks, but requires practice. The circus unicycle rider can do some pretty fantastic stuff, safely and talk at the same time. If he did it for 30 mins a year, he would soon lose the ability to do it as well.
I have seen many Captains try to fly a visual approach with the heading bug and go through the centreline a number of times on the way down. Much better to unplug the AP and point the aircraft at the right place.
We fly to Grand Cayman and at the moment there are no electrical aids at all, except lights. (might have changed by now). Some Captians are scared of this!!!!!! OK, you need to break cloud above MSA and if you can't get in divert - that is sensible. But to be frightened of it on a gin clear day (like that part of the world generally is) I find interesting.
The only reason some of these things above are considered dangerous is because we never practice. In the olden days (!) BEA said the Trident should be Autolanded wherever possible, as the failure rate for sims went up, the rule was changed. The failure rates then went down.
In BA the Airbus guys cannot disconnect the autothrottle. One of my mates has just passed the Airbus course with 1 days worth of training with manual throttle. Is he safe to fly on the line when the AT fails - or indeed to use the MEL 3 day autothrottle inop section?
If you don't practice these things, you may be OK to fly without them, but add something else like a garbled radio call and all of a sudden something like an alt bust becomes iunevitable as your capacity bucket is full. If you practice enough, added extra things do not approach a dangerous situation at all.