I've fielded Sniper drones quite a bit, (twice in causal/semi-competitive tournaments, many times in pure casual games). I think they are a lot more competitive than most people give them credit for, but their markerlights are more of a bonus than their primary function. The BS5 rapid fire sniper rifle is what makes them worth taking, that and the fact their sniper rifles benefit from Storm of Fire.
One mission in a tournament had respawning units, and my opponent had 3 Dreadknights (which did keep coming back), with the sniper drones (and Aun'va) at least one of them went down each turn (even with the 4+ invul they can get), then in the next match they annihilated 2 Tyrannid MC's in a in one turn each, again with Aun'va nearby.
In my experience, I shoot them at the biggest and most applicable threat, and if they don't kill it then I have markerlights to help my other units finish it off. Only once that I can remeber did their markerlights become their primary function, I had a unit lose all but one of its drones and still had two controllers left.
I agree that the reason they are not seen too often is that they come with the price tag (in $) of a model that has D weapons, almost anyway. Also then you can't take the 3 Broadside teams that some people like to, which I don't (my broadsides don't like me
). But sniper drone teams have earned a place in almost every one of my lists.