Reasons for frog are two fold, defence and reach. If you face a +3 to cast pyro mage with only a skink priest you can kiss your T2 naked skinks goodbye. Similarly if you face a +3 alchemy mage your scar vets and monsters are in serious danger of doing a popcorn impression and exploding. 6 turns of facing thaumaturgy at +3 with only plus one to defend means your opponent will get lucky at some point and ruin your day.
On the flip side druidism has zero offensive potential and the offensive damage from shamanism is highly situational and requires getting two specific spells with only 3 rolls. The entire SA army has a range of 18"". The slann with 3/4 paths can be that offensive reach to hit enemies at 24"-36" and even more with the telepathic link. He also allows the shoring up of weaknesses (anti armour for monster mash or numbers of low quality troops for vet spam) and the amplification of strengths (pyro with heavy shooting).
Personally I am in favour of the vast majority of lists having as slann as the iconic all powerful magic using toads were a large part of what drew me to the book way back when.
Dont get the temple guard or bow skinks though... Guess we all have more to learn about how to make the book work. Would love results for each list.
The 2 points I would query are about the scar vets, they have access to Fireborn which can be chosen as a counter to both Pyro and Alchemy. The other statement is concerning Bow skinks. They can have their uses. I tend to use 2 units of 20 with FCG, at 290 pts slightly expensive but when used aggressively they can give extra combat res if properly supported. Yes - I know - even more expense but can pay big dividends.
I am sorry I missed your last visit to Dublin would love to give you a game on your next visit if you can fit one in.