one other reason for the lines could well be a temperature gradient in the printer itself.

The reason abs isn't well suited for 3d printing is because it has built in shrinkage - this is to allow easy removal from injection moulds - which is what it was developed for..
So as each layer cools, it shrinks, the hotter the model stays the less the individual layers shrink.
The shrinkage can causes layer lines and also is what causes the really crappy layer adhesion.

Obviously the layers nearer the printbed are being heated by the bed itself, the layers further up are warmed ONLY by the internal temperature of the printer itself. And unless you have a thermostatically controlled temperature in your print area, it's always going to be cooler the further away from the print bed you get.

I also have a shelf full of abs - at some point I'll either give it away (I have tried, nobody wanted it) or dump it.
For me it's purely down to the smell and fumes.
Even on an open frame printer, if you print slowly and hot, you can get decent prints. They will not be as strong as prints with a material that has a lower thermal expansion, but they are useable.

But I have a relatively small crowded workshop and having it stink like a chemical factory is a bad idea.

Pla either doesn't smell or smells like maple syrup :-)