The situation in question sounds as much as a daylight problem as a tree problem.....
As answered, obviously, it can be both. Overall, they probably help most situations, but can hurt on a few.
Many old courses really have a hole spacing problem. More space always helps, more trees may not, at least for several decades. Average players still hit the ball to an apex of about 90 feet, and most trees don't get that high. Obviously, its lower when off line and not flush, so maybe 50-70 foot trees, before or after the apex help a lot.
Others have a "Zone of play" problem, whereby tees or greens are within 17 degrees of a hook side line of play, or 23 degrees on the slice side. Typically, I only worry about high concentration areas like that. The odds of someone passing in a ball flight in a larger fw where golfers are scattered go way down. That said, I recall Ted Robinson saying that he always staggered parallel primary landing zones beyond the driving distance of the other, which I try to do, but it is sometimes impossible with parallel holes.
As to vision, my mentors always routed side by side holes in opposite directions so players could see what was coming at them. Other architects would occasionally route them the same direction, a la Inverness 1 and 10, which they never did, even if it would look cool and make a better hole, in the name of safety.