Quite agree, but out on a road you can't do this (let them stand and look) if causing an obstruction. I also agree horses will follow you over a tarp ~(they all do), but I think the idea of doing it before hand is to get that initial "looking" out of the way and to increase trust and make such things as flapping bags more familiar. There is no harm in it and if it makes taking your horse on a public road a fraction safer for everyone then it is probably a good thing :)
If you are handling your horse on a several times a day basis and not underexposing it to anything then it will have already got used to things flapping around by it - you shake your rugs/blankets before you put them on, wipe it over with a cloth, put our own jackets off and on, we empty plastic bags of shavings into the stables when the horses are in there, nothing is done conciously to get them used to these things, its just routine that they have to deal with.
My horses are far more likely to 'jump' when a tiny little bird flies out of the hedge than they would at a plastic bag. To rely too much on 'desensitising' and not enough on trust and obedience to commands is not always a safe idea because its impossible to cover all the options.
Horses were on the roads way before cars were and I have no problem whatsoever holding traffic up if I'm riding a young horse out or accompanying someone on a young horse so it has a buddy to 'lean on' for extra confidence and it suddenly decides something is too much to deal with. If it offends a driver they can say all the rude words they like - it just goes straight through me. The safety of me and the horse is my number one priority.
The UK Highway Code has pretty strict guidelines on how drivers should behave around horses and making some driver slow down or stop for a few minutes is no big deal to me.
One of the main causes of spooking is where the horse is using an object as an excuse to get out of working (we have a monster who lives in the corner of our menage) or as Foxhunter
said as an excuse to have a leap and a buck because they have too much pent up energy when they first set off.