The larger your IP network, the harder it is to keep track of which IP addresses are in use and which IP addresses and subnets are still available. Even if your databases and spreadsheets seem to make things clear, you still have to know whether the same information is stored in the DNS.
And if you use DHCP as well, moving departments or integrating new network objects can become an administrative nightmare, because information has to be updated in several places independently and separately.