Absolutely. Although the cert does provide a higher level of security, going HTTPS is also a prerequisite for the HTTP/2 protocol. As long as they're with a hosting provider with the hardware to support HTTP/2, I would definitely do it. If their hosting provide doesn't support it, then I would suggest finding a new host.
HTTP/2 helps with improving page speed by allowing more documents to be transferred from a server to the user's browser than the previous HTTP/1.x allowed. I think the max resources per domain was 6 at a time with HTTP/1.x, but that cap is removed with the new protocol. More explanation can be found here. HTTP/2 uses multiplexing, which wasn't available to transfer resources on HTTP/1.x
So without going into a crazy amount of technical detail what the benefits are (do the reading in the link above), it's very worthwhile to go https for all sites.
Many hosting providers have either free certificates or paid certs. The paid certs are usually wildcard certs, so the encryption can be used throughout all subdomains. The free certs are normally limited to one subdomain (www). Paid certs also can move you to a dedicated IP address (because they're wildcard maybe? Can't remember the reasoning).
Long answer, but to sum it up... going HTTPS will improve site security and site speed. More security = less chance of getting hacked and more trust with users (and Google). The faster you're able to transfer resources from the server to the user means the page will load quicker and users will get what they need sooner.
Hope that helps!