Is there still a valid use-case for wildcard certificates when using Lets Encrypt? AFAIK wildcards were used for financial reasons and laziness (since the traditional method of acquiring a cert was cumbersome), but with LE none of those arguments make sense.

Why not just fetch a different cert for every subdomain you use? It's also better security practice as this allows you to use different key per subdomain and the possibility to revoke bad or unused certs.