This is what is known as Moving the Goal Posts. You specified a wizard popping up and down in the middle of a field and I supplied a tactic which completely negated that tactic. Moving the goal posts to walking behind a large tree, wall or pillar doesn't get you very far.
My error. I thought you were discussing something else. My reply was to your reply which was to someone else's reply to a post of mine. I got confused in that tangle.
Especially when a different tactic like walking right, left or around so that your cover is gone negates those tactics.
Sometimes. Typically not at longer ranges unless the cover is fairly narrow.
A 15 foot wall perpendicular cover outdoors with foes 100 feet away means that they have to walk 70 feet at a 45 degree angle so that they can attack 3/4ths cover instead of total cover (if using squares). That takes a minimum of 2 rounds for most foes, and once they get in position, the wizard gets +5 AC for 3/4ths cover.
Or, a good sized room with the wizard taking cover outside of it 10 feet to the right or left of a doorway means that the bowmen have to move through the entire room and typically the rest of the party in order to get outside the room in order to attack the wizard (sure, the scenario might have other ways around the room, but that typically means one or more rounds of movement which helps PC action economy).
The terrain in most of the encounters in the games that I have played (and most of the encounter in adventure modules the WotC put out) have some type of cover available. Even if it just fellow PCs.
As for the "bob up and down" scenario. Sure, the bowmen might eventually start readying actions to fire at the wizard when he stands. Once the PC figures out there are bowmen or that they are targeting him, he doesn't have to stand up. In fact, he never had to stand up in the first place (this might only be a good tactic if he is using a to hit spell like Fire Bolt). He doesn't have to stand back up right away. He can cast saving throw cantrips (or higher level save spells) from the ground at no penalty and only needs to stand when foes get close enough to get into melee range soon.
My point isn't that any give tactics for certain specific scenarios are a great set of tactics. It's that NPCs tend to have very few options whereas PCs have a lot of options. Versatility tends to win out.
And because PCs tend to have a lot of options, it means that WC is mostly a waste of time for anyone who isn't often in melee combat. If your wizard PC at your table is attacked in combat a lot and isn't designed to handle it (like my plate and shield cleric/necromances), then sure, WC might help. But, better tactics and spells would probably help a lot more without using up an ability score gain for a feat.
If a low level wizard PC casts Web and gets 3 foes and has no cover in a field, he's better off Dodging for a few rounds (or using the drop prone trick if he really wants to cast some more). Unless it is a big fight, it's doubtful that most wizard players will be doing anything more than throwing some cantrips anyway. Will the DM sometimes throw a Fireball on the low level wizard prone on the ground? Sure, maybe. But if the low level wizard is taking a Fireball to the face, he's got bigger problems than keeping a concentration spell up.
WC doesn't prevent the wizard from taking damage. Dodge, or dropping prone, or total cover often does. Tactics (and spells) over once in a blue moon abilities.