Well, that's not exactly it.
Actually, you *could* change the partition setup of the drive. But then you would have to reconfigure all running daemons (services) to take into account the new configuration (samba, nfs, twonky, trustee, etc...) so you can use the new drive.
Also, there is one major risk: skrewing up when messing with the partition setup. Indeed, the current hardware setup is weird, to say the least. The MBL has a RAID setup with two of the disk partitions. And you get a warning from the kernel that this is useless and will not protect you in case of total disk failure.
To throw some suggestion to avoid reconfiguration of the sharing services:
mounting one of the partition as /shares/xxxx or /shares
using symbolic links...
Indeed, if one adds new partitions, and mounts them as
one actually achieve space quota. However, these quota are written in stone. So, maybe useful for timemachine. But not-at-all versatile for users. Also, it lacks scalability.