I realized that one network I'm connecting with is using a MAC address of one of its interfaces.
If an attacker would know that fact, could he somehow obtain the MAC address wirelessly and break into the network? Let's exclude brute force attacks.
The answer is: No it's not a good idea.
There are several reasons for that. The first and most important is that the mac address of the router is broadcastet by the router multipile times every second. This is done by sending so called "beacon packets" which contain a log of information which is realted to that access point. This means the mac address can be obrained by simply running a tool like
airodump-ng which captures and displays the content of these packets.
Also it is never a good Idea to use a password that has a direkt relation to the service it is used for or the person the account is associated with. This is because its very common for attackers to create wordlists from every pulic available source that is related to the target. This might also contain a scan of the wireless networks and in that result the mac address of the accesspoint.