It makes sense to use ECDSA for leaf certificates, because the TLS server can then handle more clients compared to a RSA based certificate of the same strength (the private key operation is much cheaper with ECDSA and is needed for every TLS handshake). The client of course, needs a few more cycles to verify the signature, but that is not noticeable most of the time.
IMHO it does not really make sense to use a ECDSA root certificate unless you have a very constrained environment, where every byte counts. The root certificate will never be transferred to the client during a TLS handshake - so the size benefit is minimal (the intermediate certificate will be a bit smaller, because ECDSA signatures are smaller). But the signature validation will take more cycles on the client in every TLS handshake.
Other than that it is a good thing that Let's Encrypt now has an ECDSA root. When researchers might find a problem with RSA in the future, we have an alternative ready to use.