I know it's "bad" but I always put big apps on my data partition
Yeah, I should probably re-install and re-partition things... When I tried to set up my partitions manually I kept getting an error that partman failed, so I went with the 'use the whole drive' option to get going.
It has to do with the hierarchical file system, as I understand it. I always thought of /opt as being "/optional" - the idea that anything going in /opt is fairly distinct from the rest of the root hierarchy, in that anytime I've installed something in opt I've ended up with an /opt/lib and even /opt/etc - distinct from /usr/lib or /lib or /etc. If the time comes to remove the package it shouldn't interfere with the rest of the system.
Thanks for the clarification - that would certainly make sense. Coming from a Windows background I have a lot to learn/unlearn and factoids like this help a lot