I can think of three reasons not to unsubscribe. The first is that it takes effort. If you believe that the effort it takes to unsubscribe is greater than the effort to delete all future academic spam, then sure, don't unsubscribe - it's your call.
The second reason is if the spammer actually is malicious and is attempting to phish for information. This is what things like Mimecast is for, which I'm not an expert on, but presumably your email service provider will already have some kind of defense in place. I would trust them to work. If the service provider flags the unsubscribe link as dangerous, then I would not unsubscribe.
The last reason is that you allegedly tell the spammers that your email is being monitored and read. But if you are an academic, then your email is already known to be monitored and read (a lot of your published papers will have it, it's available on your department's website, etc.). It's probable that the entity sending the emails got your email from such avenues in the first place. You can't both publicize your work and avoid revealing your email address without going to extreme lengths, so not unsubscribing doesn't actually do anything.
A final point is that if you don't tell the entity that is sending the emails that you don't want to receive them, you can't really fault them for continuing to send it. E.g. I've gotten emails ("spam") about open research/faculty positions that I am overwhelmingly unlikely to be interested in owing to personal circumstances, but I can hardly blame the sender for not knowing those personal circumstances. If I unsubscribe and they continue to send those emails, then they should know better, and it's a different matter.