Leaving the first comment on an event, forces you to acknowledge it. That's probably not what you want.
Right now, even guest users can acknowledge events. That could have unforeseeable consequences, as acknowledgement status is relevant in two areas: Filtering and escalation. On the other hand, consider a 3rd person, who has access to Zabbix and can see an event as a guest user. While this person is not responsible or capable of solving the problem, it could leave a hinting comment on what caused the event.
While I see that Zabbix is not an issue tracking system, I see some use in allowing comments without having to acknowledge an event. If you don't consider it useful, guests should not be allowed to comment, in my opinion.