WARNING: When facing these creatures, it is vital to kill/disable them before they can hit you. With the exclusion of Evolution, who can restore herself by transforming, this creature's default attack will PERMANENTLY lower the target's Mana, with no way of regaining the lost Stats.
In battle SuckMana is a dangerous ability to face, while on allies the crippling factor is less meaningful since you won't see those enemies ever again. However it also allows the unit to sacrifice their max Mana to increase Magic outside of battle. While the rising cost means there's a limit, it still gives them a means of powering up beyond levelling.
While they do have all four skills fixed, without a single one that uses Magic, it's possible to get them spells through skill-focused fusion at the Farm. Even before then, you could get them Telekinetic or use them as Combat capable Mana batteries, though that also becomes less meaningful once you get the Transference Essence.
They have Glowing, which removes Vision penalties, allows access to shadow biomes, and reveals full enemy party compositions.
Worms help greatly with mining.
Since the best way to use them is getting them spells via fusion, you might not want to bother breeding them.
They can be sold for 100 gold at Medium size and for 300 gold at Enormous size. Every level above 1 additionally increases their price by 5%.