(2) Disadvantages. The disadvantages are:
(a) Shorter fields of fire.
(b) Enemy can approach in cover.
(c) Enemy will have a downhill assault.
(d) More manpower is needed to patrol, observe and cover dead
ground.
a) No problem, for most defenders do not need to open fire at long range - that's the job of snipers, mortars and artillery.
b) Not true unless you omit observer positions on the ridge.
c) No problem, that's what barbed wire is for.
d) I don't get why.

An anecdote (recalled from memory, slight deviances from the original story are possible):

An U.S. army platoon dug in in a defensive position overnight sometime in 1944. It was on a froward slope.
The next day, German observers detected the new position and phoned them to Bn HQ.
There was some unidentifiable noise over the next night.
The third day began calmly, but when morning fog had cleared, a camouflaged assault gun began opening fire on the forward slope position, decimating the platoon. The Platoon 2nd in command finally decided to evacuate, but when he did so many machine gunners and snipers opened fire. Almost no-one escaped.
(The same effect could have been had on the second morning already if light infantry guns were used or even on day one if modern bazooka-type weapons were used, but the assault gun provided near-immunity to OPFOR mortars.)


Ridge position are for observers,
ridge (and forward slope) positions are for (daring) snipers.
reverse slope positions are for the counter-attacking force,
counter-slope positions and 2nd ridge positions are for the main force.

You can differ, but a capable opponent will punish you terribly.