I like the use of a single tree to create a dogleg. I don't like the use of a single tree to create a barrier for an approach shot.
There's a venerable course in the Twin Cities called Keller, where the St. Paul Open was played for decades (Hogan, Snead, Nicklaus and Palmer all played there over the years) that features two holes with fully mature oak trees growing directly in front of the greens. One's a par 3 of about 160 yards, the other's a medium-length par 4. A normal-trajectory approach shot with a 7- or 8-iron will clear either tree, so they are not physically prohibitive barriers to the green (although a mis-hit, obviously, will nail the trees); but they are certainly mental barriers -- especially to higher handicap players -- and they do eliminate the possibility of playing a low shot to the green from straight-on.
I don't get the point. This course has plenty of trees to look at; the fact that two of them jumped out of the woods and took root in front of greens may seem quaint to some players, but ultra-gimmicky to me, no matter how old and stately the trees are.
A new course in our area features a tall, slim tree about 40 yards in front of one of its greens, and though it's a lovely tree on an otherwise mostly treeless course, it doesn't belong where it is, blocking all of the green from the right side of the fairway.
I can think of many individual trees I've hit too often in my golf career, trees that I'm not on good relations with, trees my ball just can't stay away from. But they all belong where they are, and it's my job to avoid them. The three trees I've just mentioned ought to go.