Wouldn't it be more efficient to harvest trees one at a time then?(or at least in a pattern where they won't overlap) This would prevent bonemeal being wasted on a "bad" tree gen.
Indeed it would, but it really depends what the priorities are. In our case, we have essentially unlimited bonemeal, and need lots of coal. Also, I expect nobody wants to spend any more time than they have to growing and cutting down trees, so having them in an easily-manageable, compact area where they can't grow tall enough to become epic trees (due to the glass blocks placed 7 meters above them) is important too.
If your supply of bonemeal is limited, you want to maximize the ROI of each piece, and you have time to spend running between individual trees, then by all means, make a tree farm where the trees are separated by a space of 6 blocks or so in all directions and have at it.
EDIT: I should also probably mention that having the trees grown up against each other like we have them reduces the number of saplings considerably, since they fall from leaf blocks, which are now shared between the trees. Instead of a standalone tree having around 50 leaf blocks, each tree in our farm only "has" 10 or so. I find that this prevents the over-abundance of saplings that one would normally get from a farm consisting of standalone trees... indeed, at times, it seems like the tree farm pattern we currently have comes close to not dropping enough saplings to fully replant.