True but if your MAC just happened to match a MAC the litigating attorney had on file due to the exploits which ChuckMcM is referring, it would be much more difficult to claim innocence as you can do with a shared IP.
MAC spoofing would only be relevant if you were regularly using random spoofed MACs on your own network.
Not necessary -- someone could spoof their MAC to match mine. IMO, this is more likely when there is already malicious intent, and it's almost trivial to do.
I've used spoofing frequently on a local college's campus to get around blocks they've put on my system. Apparently some of the student use Ethernet jacks aren't for student use.
The case for MAC spoofing being common (at least with people "borrowing" your wi-fi) makes sense because some networks have MAC address based white lists, so this defense isn't quite as crazy as it sounds at first. IANAL etc.
MAC spoofing would only be relevant if you were regularly using random spoofed MACs on your own network.