While a base 15 ("Bring a plant to maturity in a single day or night.") D:Sun CrHe spell would, I agree, be pretty pointless if the plant reverts at the spell's expiration and your purpose in using it is to have a mature plant, but:
- You could have another purpose, such as having the plant grow in such a way that it destroys a structure as it matures. The structure will remain damaged after the plant reverts to its original state, so the purpose will have been achieved without requiring permanence.
- You could use a higher-level base effect or a longer duration, resulting in the plant spending some period of time in a fully-mature state before it reverts. (e.g., base 30/D: Moon would mature in a Diameter and then remain mature for at least a couple weeks before reverting)
I do agree that the most obvious interpretation is for "maturation" CrHe effects to be permanent without requiring the expenditure of vis, but I also think that it opens up additional interesting possibilities if they're temporary and expire when the spell ends. Being temporary also feels more consistent with other Creo effects, although it does open up some new areas of ambiguity that would need to be resolved. (e.g., Say you mature a tree with D:Moon and, during that Moon, the tree is cut down and used to build a house. What happens to the house when the spell duration expires?)