The tree is competing with your attack providers, not sentinel outposts (which still remain the top building if you *just* want more defense). In general I consider 1% attack about "equal value" to 2% defense. So 1/1 + a FP seems fair enough in comparison to a sentinel. 2/2 + a FP in AF+ blows it out of the water - even including the road.
The neat part about these trees including roads are some of the visual appearances of cities that can be justified as "right" - where you'll get some heavily tree lined roads with access roads behind them : i.e. with a road servicing 4x4s:
Code:
4 x 4 4 x 4 4 x 4
4 x 4 4 x 4 4 x 4
4 x 4 4 x 4 4 x 4
4 x 4 4 x 4 4 x 4
T T R T T R T T T R T T
R R R R R R R R R R R R
T T R T T R T T T R T T
4 x 4 4 x 4 4 x 4
4 x 4 4 x 4 4 x 4
4 x 4 4 x 4 4 x 4
4 x 4 4 x 4 4 x 4
Can make for some very aestheticly pleasing cities while averaging only 1 extra piece of road per 3 trees (or even less if for instance it was 5x6 buildings then it'd be 1 piece of road per 4 added trees).
Without the road requirement, it'd be wrong to place them as such.