As someone who is a modeller first, gamer second, I instinctively dislike any rule which disincentivises dramatic posing and scenic bases. For me this is why this type of rules question will always come down to player discretion.
As a narrative gamer I like to fill in the blanks, so I'm not moving my character 5" up to that terrain piece, he's running forward before taking cover behind that bolder and there's no way he holds the same pose as the model throughout the entire battle. For this reason my group plays 50% obscured = cover, 75% obscured = not visible, with players being sensible and a roll of if we can't agree. Likewise since 2-4 trees doesn't constitute a garden, much less a wood, we prefer to imagine minis moving through thick, sentient undergrowth which my attack at any moment. For this reason, woods block like of site to those behind them and give cover to those inside.
Another point about area terrain is that it spoofs players taking ages to exactly place their model to get the benefits of cover.