It's very strange, because this check very easy to implement. Same with casting spells on units with immunity, can't imagine why this still present in the game.

I've seen spiders choose not to web my archers in .85, but they still 'overkill' web/beguile on to a single unit, rather than spreading it out.

Webbing ranged creatures ain't always a bad thing, I do that when I want to make sure that my summon will hit the unit and beat it up to a pulp instead of chasing it around (which is easily prevented by AI sending a melee unit next to yours to limit movement while pulling his archer / caster away).

Regarding AI, I haven't noticed lately any double web or other effects of same class cast / used on same unit.

If beguile could stack and be for ranged and melee units that would be a workaround, I guess.  But I had a double beguile yesterday in 0.85.

With the brood archers now in the game, the spiders could do a web wall, protecting their ranged units for a few rounds with a sticky wall that lasts for 5 rounds, or you have to hit it with two melee units that are then stuck for two rounds to open it for other troops.  

It will only last 2 rounds if you set fire to it with a burning hands, fire staff, or burning axe.  That would be cool.