I'm assuming the question is asking why should you add these services to a network that is already stable without them. DHCP adds scalability to your network and makes address management much easier; rather than having to manually assign addresses to each device, you can have a DHCP server assign network addresses to devices, and provide them with additional information like the default router, ntp server address, etc. DNS makes it easier to access network resources, as when running a local DNS server you can use hostnames to access network devices rather than ip addresses. When running DNS and DHCP on the same server it's usually possible to have the DHCP server service automatically update the DNS server with the ip addresses assigned to a host with a given hostname when its address changes or register that host the first time it is connected to your network, this usually further reduces the amount of management the network requires. A local DNS server that caches frequently resolved global hostnames can also reduce traffic on your WAN links by reducing DNS traffic travelling over those links. An NTP server that synchronizes time across your network is important because some protocolls, like the kerberos authentication protocol require timestamps when authenticating, and if your devices are out of sync authentication can fail. Typically, computer motherboards have a built-in hardware clock with a quartz time-keeping crystal. These almost all suffer from varying amounts of clock skew, so even if your network appears in stable and in-sync now, its highly probable that you will need some sort of NTP server.
As to your second question: "Why is wireless networking seen as a nightmare," I can't speak on behalf of the whole IT community but I can tell you why I hate wireless networking. My primary issue with wireless networking is that it poses a security risk to my network. Sure, it provides convienent network access, but unless ALL access points are properly secured the wireless network opens my otherwise secure data to all sorts of eavesdropping and interception risks. Granted it's relatively easy to set up WPA2 personal security, but this doesn't provide detailed accounting logs which are critical in enterprise networks. Which leads me to the second reason I hate wireless networks, it's actually quite time-consuming to set up an enterprise level wireless network, and there are usually many more important tasks I can be focusing on at any given time than providing my employees with yet another way of accessing the network they already have access to. But really, my biggest concern is security.