I'm assembling a bike for a friend and it comes with D/A 7800 shifters with the attached adjuster for rear derailleur. Rather than the fine adjustment on the other stop/adjuster it has five detents. I don't quite understand the logic of such a limited range. Any insight?

it for fined tuning of adjustment when the bike is ridden .

I understand that, but I can do that with a normal barrel adjuster. This makes it easier?

That type of cable housing stop came with Shimano's higher line groups, Ultegra and DA, while the lesser stuff, 105 and below, made do with rear cable stops with real barrel adjusters. Why? Dunno. I guess the thinking was you would do your fine adjustments with the derailleur mounted barrel and these just gave you a quick fix if something was really out of wack.

If your cable is properly adjusted, this will give you sufficient adjustment range. I set my bikes up one notch away from slackest setting, then the following notches will take up sufficient slack, should it be necessary as you ride.

Adjustment should be made at the detailers. Those inline's are for on-the-fly tweaks.

Yes, the key words are on the fly tweaks. Not initial adjustment, not for major adjustments. Just tweaking for little noises and sluggish up or down shifts.