Plague of Toads is quite an interesting ability.
- Very High Damage
- Very Cost effective
- Applies poison damage (for BadMedicine)
- Very short ranged/Unreliable even at medium range
- Expensive for a Primary (not sustainable without some sort of mana boost)
The runes generally change one of these properties - Toad Affinity "fixes" the mana, Rain of toad fixes the range, Explosive toads sacrifices the poison in exchange for more damage.
The key question is "When would I use toads over poison dart or spiders?".
Compared to Both Spiders and Poison Dart, toads have lower single-target damage, and much shorter range, so toads are a better option if you're able to be in the think of it and have enough damage for priority targets in your build and have some slack in your mana needs. In exchange, you get a lot more damage into groups.
If you can meet these requirements, Plague of toads will put out A lot more damage - you can reliably hit 3 targets in melee range, putting out 390% dps unruned to a splinter dart's 180%.
It synergizes well with Pet Builds, and with Bad Medicine (and Jungle Fortitude as well), which provide the sort of tankiness you need to stand toe-to-toe and spam toads and are easy on the mana. It also synergizes with Locust Swarm, which has similar strengths and weaknesses, Haunt provides some complementary single-target damage and Grasp helps bunch things up so you can land more toads, and with Soul Harvest/Fetish Ambush/Mass Confusions because you'll be in the right position to get a lot of value out of these PBAOE abilities anyway.
It doesn't synergize very well with Horrify, because the spell breaks up your packs of targets, nor with spirit barrage or acid cloud, because of the mana concerns.
Overall, it is a lot more awkward than darts and spiders does need a lot more support from the rest of your build, especially as the enemies start to hit a lot harder, but it does have some interesting strengths in specific sorts of builds.