8th Ed. Jungle swarms. Can they be worthwhile?

Just adding more bodies to a unit often isn't a great return on investment. There's a diminishing return. If they can't win combat, then it just means they may stick around for one more turn but you lose more points when they die or break.

There are certainly situations when adding swarms into the mix doesn't help and can actually hurt. But combo charging or counter charging with swarms can allow a saurus block to defeat a unit that would otherwise grind them down. Off the top of my head, swarms have made a positive difference for me in combats between: (1) saurus warriors and plague bearers; (2) cold one riders and beasts of nurgle; (3) and most memorably in a game against Tomb Kings the poisoned attacks granted by 2 swarm bases allowed a unit of Saurus warriors to run roughshod over a Khemric Titan and then a War Sphinx.

In order to get the best effect from the poison, you obviously want to limit the number of models from the enemy unit that can attack the swarm, so corner-to-corner is a must. You also want to use the swarms in combination with a unit that has a lot of attacks. Horde formation gives you an extra rank, but opponents can use that against you if they charge your horde with a narrower unit it can make it impossible for your swarms to counter charge and join in. With a big horde, you want to set things up using your skinks as bait so that the Saurus can get the charge in conjunction with the swarms rather than trying to take a charge and stick.

Finally, if the saurus are going to get smashed one way or another, don't send the swarms in since it's just giving up more points.