A rogue access point is a Wi-Fi access point connected to your network without your authorization.

Considering that (1) a Rogue AP is not under your (or your network administrators') direct/complete control, and (2) the fact that it is not bound by your network security policies; allowing a rogue AP to be part of your network exposes your network to unknown security risks.

You are better off assuming that a rogue AP would be eventually used to allow others (anyone with a WiFi-enabled device) to tap into your private network -be it your wired LAN or your wireless LAN. A Rogue AP should be viewed as a backdoor (an unguarded portal) that could potentially expose all your LAN/WLAN based IT assets to criminals, hackers, and crackers.

Having a Rogue AP on your wired LAN is no less risky than having one associated with your WLAN. Considering that you have no direct control over the management of Rogue APs, allowing one to associate with your wired LAN (even if you do not have a wireless LAN) can effectively open-up/expand your LAN to include a new network segment that you would have no control over.

Most Rogue APs discovered on corporate LANs are usually benign -they are often installed by well-meaning employees who are either trying to improve their productivity, or trying to solve a problem on their own. Even though introduced innocently by well-meaning individuals, such rogue APs pose no fewer security risks to your LANs than the ones sprung upon your LAN by criminals.

Just as you would not open up unguarded doors on your personal or business properties, you should not allow anyone to operate an unauthorized (rogue) AP within or around your (wired or wireless) LANs.

As 802.11 technologies have evolved and become increasingly popular, wifi devices are becoming less and less expensive, and relatively easy to install. As wifi networks gain more footprint, so does the threat of such Rogue APs to your personal/corporate networks.

In order to proactively manage serious/imminent threat to your network security, you may want to have a clearly stated network access/usage policy that forbids use of unapproved network gear on and around your LANs/WLANs, communicate it to your user-base, and enforce it rigorously. Security risk posed by Rogue APs is not something that an IT manager or a Network Administrator can afford to ignore these days.