If I hide the SSID the AP wouldn't need to handle these probe requests and therefore the workload would be lower. Is this assumption correct?
Hiding the SSID doesn't in any sense block probe requests. All hiding the SSID does is to simply allow the AP to omit (or leave blank) the SSID field in beacon frames and non-directed probe responses.
The AP still receives the probe requests, processes them and may or may not respond depending on the nature of the probe request. Additionally, there is no real savings in the "airtime" used which is often the true measure of a wireless network's "workload."
The devices that should connect know about the SSID and all other devices don't need to know that the SSID is even there.
Other devices will know the SSID is there, but they won't know the name (although it can be determined when clients who do know it connect). The SSID may not be displayed to the end user, or may display as something like "".
Is it even worth to hide the SSID or are the negative side effects to big in comparison to the gained advantage (if there is any)?
In today's deployments, the only place I personally find hiding SSIDs potentially worthwhile is when you have a situation where customers and staff/employees may be connecting to similarly named networks. In these cases, hiding the SSID of the staff/employee network may make it less confusing for customers.
Although even in these situations, often choosing clear SSID names is better than hiding the SSID in my opinion.