This is my main takeaway. I suspect the vast majority of iPhone users either do not secure their phone at all, or use a 4-digit PIN, and therefore are not protected against targeted attacks anyway. Touch ID can improve security for these users.
My main reason for protecting my phone is to prevent a scenario where it gets stolen, and the thief can access my data with reasonable effort. My secondary reason is to protect against snoopy acquaintances. A 4-digit PIN marginally achieves these goals: a typical thief is locked out, but a more sophisticated one can easily brute-force my code using commercially available tools.
What if I use Touch ID instead? It is very likely that a thief will not know who I am, and therefore will not be able to go around lifting my fingerprints. They might, however, try to lift prints off my phone. The way to protect against this is to only unlock the phone using my thumbs, because (per TFA) there are typically no good thumb prints on the phone itself. If I do this, then I will in fact be more protected than I would be using a 4-digit PIN.
A couple of caveats. After 5 rejected attempts the iPhone will fall back to asking for a PIN. It is therefore advisable to set a strong (longer than 4 digits) PIN here. Second, this of course may change if the 'secure enclave' that stores the fingerprint hashes gets hacked.
TL;DR: If you only use your thumbs to unlock your iPhone, Touch ID currently provides better security against typical threats than a 4-digit PIN.