Value Streams are designed for thing-driven models.As per description of value streams it store data from an associated thing’s property.
A value stream defined at template level stores separate data for each Thing implementing this Template ,so purging the value stream at Template level will remove value stream data for all the Things.
Imagine a situation where two user(user 1 and user2) are using a same Template having a Value Stream to store data for their respective Things and if user1 purges the value stream data for its own Thing using service at template level ,then this will remove the value stream data for Thing created by user2 as well which is troublesome.
So I think it’s not a good idea to purge value stream at template level.
Hope it makes some sense..!!