I strongly agree there is no reason to join the Cult.
The main predicament you are in is the tadpole and the threat of dying due to ceremorphosis. It is never mentioned or even hinted that joining the cult would prevent or forestall this condition.
There are also no other motivators, some of which could be:
- Monetary reward
- Mention that joinging the cult would give you power
- Some of the people at the grove have slighted you
