>Why reduce sail area?

its merely a consequence of a more efficient sail
and the need to keep weights down as that affects the entire rig, and costs speed
the sail ought be designed for an individual boat, ie no more area than required

most experiments seem to show that 50-60% of normal sail area is a workable sweet zone
lift is not necessarily less, but it does mean that very low windspeeds might be less competitive
and this is what is keeping wings out of competitive use

if a wing could be devised that, its area or geometry could be variable in area, there would be few other impediments other than weight

> Given a well proven ability to quickly reef and/or depower, why not have more sail to make the most of low winds

reefing is the principle issue for which wings face, as it is not their forte, in no way is it proven
personal opinion is that this factor can be overcome, simply a matter of time on design

> Does the extra weight of a soft sail matter?

assume you mean soft sail wing
not especially, but its not in the best place and ultimately costs speed

at this point in time, a single surface sail is probably the cheapest and best average preformance,
but the wing sail is coming.