The only thing the trickle charger may hurt is your wall outlet and its own plug, watch both like a hawk for any signs of heating or blackening of the terminals. If you find any, replace your wall receptacle. To produce a useful amount of 400V DC current for the Leaf battery from a 110V AC source takes more than a "trickle" of wall current.
Aside from that, the charge rate from the trickle charger is barely enough to heat the battery at all, and it's axiomatic for every battery chemistry known to man that slow charging is always healthier than fast. Some battery types are hurt less
by fast charging, but that's all you can say. From a longevity standpoint, if you use the car little enough that the trickle charger keeps you rolling with no inconvenience, then it's the healthiest way to go.