From an IT management perspective, setting up a DHCP server would really depend on the size of the company. A DNS server could be recommended in any setting for faster browsing and recognition of sites. If your network is running the same Operating systems and it uses a name resolution method other than DNS, you can continue to use that method without needing DNS. If your network is running different operating systems, or it connects to the Internet, you will want to deploy DNS as the Internet consists of protocols that require DNS.
Justifying setting up either server really depends on the admin, but if you think about it managing two computers statically is nothing compared to having to mange twenty computers statically. Having only two computers set up statically on a network will not be very time consuming having to change the setup if something on the network is replaced. Assigning client addresses automatically is by far the easiest option of the two.
When setting up a DHCP server and leaving room for growth on the subnet you use, can also save time when installing new computers on the network. It would save the time of manually having to configure any new computers. Not to mention it takes way less time configuring a DHCP server than it would to configure each computer statically. Obviously you would want to pre-allocate IP addresses for fixed hosts such as routers, servers or even printers. These you would set up on the DHCP server as reserved addresses and set them statically on the devices.