Take offs, climb outs, descents and approaches in busy terminal areas are about achieving and maintaining a good situational awareness, and automation is a great tool for that.
A good sit awareness during those phases in non busy airspaces require much less "brain megas and megahertzs". Hand flying cannot reduce your situational awareness. It can even improve it, actually.
If you have to handfly in the busy terminal area (automation u/s) you will either do it fine without losing any of your situational awareness or do it with less. It depends on how much you have practiced.
Besides, it is the crew situational awareness that matters. Not only yours when you are flying (hand or automated). So we should be used to practice both handflying and assisting a hand flying PNF.
How can this be dangerous?