Slaad Chaos Phage seems like an odd ability in 5e. They offer no real disadvantages to the party that forces them to expend resources because they require days to take effect: Red Slaad Tadpoles kill whoever they infect in three months while Blue Slaad plague causes maximum HP reduction per day until the target is turned into a Slaad.
In short, as far as I can see, a Slaad infection doesn't seem to increase the fighting proficiency of the Slaads in any way.
Which then brings up the question of how are they supposed to work towards increasing the difficulty of an encounter, since Slaad infections are diseases, and Lesser Restoration is a fairly low level spell.
- A party without access to it is unlikely to survive the Slaad in the first place
- A party that is sufficiently high leveled to take down a Slaad would probably have lesser restoration on hand, which they can cast and then long rest to regain all their spells
The alternative would be keeping infections a secret and the party suddenly wake up one day with a member-turned-Slaad trying to eat their face off. This though seems to be a power move with no counter and extremely unfair for the party, especially for Blue Slaad phage since it means one character is automatically removed from play with no hope of reversal unless someone has Wish handy. At least someone killed by a Tadpole can still be revivified.
Wouldn't a Slaad infection that causes creatures to turn before the day ends or even mid encounter (A la XCOM Chryssalids) be a more sensible flavor ability? This infection would force players to expend potions/spells slots in the encounter and serves a point to effectively deplete player resources.
Best Answer
Not all monster abilities need to be directly combat-related
First, you assume that the characters would know the details of the infection. They would likely realise they are infected by something, but the gradual reveal of its nature can be played for a certain amount of horror.
And just because Lesser Restoration is a relatively low-level spell (compared to the probable level of the PCs at this point) doesn't mean they will immediately have access to it. Perhaps there isn't a cleric in the party and they have to get to someone that can help? Or the cleric hasn't prepared it so need to risk waiting until the next day with no way of knowing how long they can last? It all adds to the tension!
And, as I stated at the beginning, not all creature abilities are necessarily related to direct combat, but can be used as role-playing or storytelling hooks by the DM.
Imagine coming across a small village where many people are getting sicker by the day. Then other healthy people start getting murdered by something creeping up from the sewers at night and...where did those sick people go?