Well, it depends on whether wlan0 and eth0 are on different networks. If they are, then the answer is yes, and you are screwed.
If both interfaces get the same ip, then you can maintain persistent connection. For example, let's assume that you constantly switch between different interfaces (wlan0 <--> eth0), when you move between buildings on campus.
In the latter case, you can bond wlan0 and eth0 (bond0 := wlan0 + eth0) and use bond0 in all your networking scripts (but still, wpa_supplicant runs on physical wlan0). In this case, nothing but the kernel cares what physical interface carries the traffic. Last time I checked about a year ago, systemd-networkd had some obscure bug in this situation, so I'm using netctl that works perfectly. If you need, I can dig out the relevant profiles.
I mostly just use LAN when I need to download a lot of stuff at home, because WIFI is much slower even at 54 Mb/s, especially since my home network is 1 Gbps. So I could just turn WIFI off in those cases, that would be an acceptable situation. However, I want to understand all of this as fully as possible. So, yes, I would appreciate your profiles - but please take your time digging them out :) Cheers, Bennett -- GPG fingerprint: 871F 1047 7DB3 DDED 5FC4 47B2 26C7 E577 EF96 7808