Any free weight exercise that involves an arcing ROM is flawed and can be made better. Just as with free weight curls, you're not getting more by doing them strict with DBs while sitting, because of the biomechanics. The beginning ROM hardly works anything. Then right after that you have a sticking point that actually forces you to stop working before the delts are completely fatigued thru the remaining ROM. You can make it less strict for sitting laterals by lowering the DBs below the bench and then cheating a little to get them started, while you lean back. Or stand with the DBs in front of you, cheat very slightly just to get them moving then let the shoulders take over while bringing them backwards slightly, up to your sides. Does not hurt the back at all if you're doing this right as you're cheating while standing straight up. As the DBs come up lean back slightly. Doing it all in strict form will not work the shoulders as thoroughly, especially at the end of a set when you can take it further by doing it this way.
To do them in strict form use cables, not DBs. Even harder on the shoulders though, have to ensure good warmup and don't make it the first shoulder exercise of the day.
Bent-arm laterals are the best exercise for side delts along with partial ROM militaries.