What's the point of disabling USB HOST? After searching and debuging like an idiot, I found out that my phone has the USB host installed but disabled. Is this the default on Android? If so, is there a security reason behind it – or what is the point here?
My phone has the USB host installed but disabled, what's the point of disabling USB HOST?

USB Host would be required to allow USB On-The-Go, often abbreviated USB OTG.  It is a specification that allows USB devices such as digital audio players or mobile phones to act as a host, allowing other USB devices like a USB flash drive, mouse, or keyboard to be attached to them.  

Security - by disabling USB OTG, a device manufacturer may want to limit support calls of third party devices or enhance security by disabling all chances of low-level commands being sent via USB port at the cost of user functionality.

CPU/Memory Performance - when a USB OTG device is plugged in, the device needs to process the new resource.  This can be loading additional kernel modules or cataloging the files for presentation to the user.  On a low-end phone with limited processing power and/or memory, a design decision may have been made that it be better to appear to always be fast rather than risk having more features, but a more laggy user experience; hence disabling the feature altogether.

Battery - a device connected will draw power from the device's battery.  Although this issue can be resolved by using a Y cable to draw the power from a secondary source.

Is this the default on Android? 

It is typically common for higher performance Android devices to have USB OTG enabled.

OTG enabled devices

http://www.corsair.com/en-us/landing/otg-compatibility-list

http://www.kingston.com/us/usb/otg

http://usbotghelper.wordpress.com/confirmed-devices/