This is still far more secure than the non-existent passcode most have been using. It's not shocking that Touch ID can be hacked, but let's get real, this isn't something most people need to worry about, and this is still a much better solution than not using a passcode, especially considering it makes it easier to use a long-passcode that is more complicated.
And for those wanting to be extra secure, let's dial down the drama level and focus on some reasonable solutions that at the very least minimize this problem:
First of all, to be completely clear, once you put your iPhone in Lost Mode with Find My iPhone, you would need to know the full passcode as opposed to being able to use the fingerprint. So enable Lost Mode as soon as possible. If the device is turned off, the sim is removed or it has no WiFi access, this won't help, but if the device eventually gets any internet access, it will kick into Lost Mode and Touch ID will not work to unlock the phone. Thus, if you have a long passcode setup, you're likely in better shape than you were with a simple passcode.
Some other things:
So what about enabling Airplane Mode/WiFi so that Lost Mode cannot be implemented?
Disable access to Control Center on Lock Screen in the Settings Menu.
What about the Siri bug?
Hopefully Apple patches that sooner rather than later, but if you're really concerned about security and want to use Touch ID, disable Siri until Apple patches this hole. I would imagine that they'd do so in the next few days, so I consider this a minor issue that won't be an issue in a matter of days.
Once these things are accounted for, Touch ID becomes a pretty reliable option for most people, as most thieves aren't going to be bright enough to crack these phones reliably with Touch ID, and with Activation Lock and Long Passcodes, they're going to be a lot less likely to try.
For those that require the utmost in security and are at risk from parties with the means to gather and replicate your fingerprint in a time-efficient manner, than obviously Touch ID is not for you, and you should be using a long passcode to protect your sensitive data. This is inconvenient, to be certain, and I'm sorry for that. I can only hope you're being paid enough to keep such sensitive data secure so as to make the inconvenience worthwhile. But again, for most people, Touch ID goes a long ways towards making your phone more secure, and we can only assume that this is just one step towards making your data more secure and that more will be unveiled in the future.
In the meantime, what are some things that would make this even more secure?
Allow users to set a shorter timeframe before requiring the full passcode. Even being able to set it for as little as an hour would make this far more secure tech while maintaining quite a high level of convenience. This is really the simplest option and perhaps makes the most sense in the short term.
Building off that, another option might be to set a timeframe that once expired, still allows you to use the Touch ID, but forces you to use one specified fingerprint or multiple specified fingerprints in a specific order for the sole purpose of unlocking the phone if it hasn't been unlocked after a certain amount of time. Also, force it to default to the passcode if you screw up the order or the specific fingerprint twice in a row. This adds more to the setup, sure, but it's an option that would make Touch ID that much more secure and convenient. And again, the bigger idea with this isn't to just allow you to continue to use Touch ID but to allow you to use a longer, more secure passcode which is harder to crack than a simple passcode without the inconvenience of having to put that in so often.
Another long shot idea would be to work out an agreement with carriers that minimizes the ability for a thief to simply remove the sim in order to cut off data. I would imagine that in 2013, they could cook up a way for the last working cellular connection to be maintained without the sim for, say 48 hours, for the express purpose of enabling Lost Mode. The only way to keep the phone from being locked with a long passcode at this point would be to keep the phone turned off for 48 hours, at which point Touch ID won't be an option. Granted, this whole idea is formed around the idea of the carriers taking on more responsibility, at which point you'd have to imagine they could do a lot more than simply turn on Lost Mode, so...meh.
The bottom line is that sure, more could be done to make Touch ID secure, but in the meantime, Touch ID combined with Activation Lock is still a pretty damn secure option for most people. Again, considering how many people don't even use passcodes, this is major progress.