At least for proprietary apps, security is about trust. Given their other slip ups in the past, along with present design choices like making certain administration tasks available only through the web interface, I wonder how people can trust LastPass.
Of course, it's better than not using a password manager at all. But one has to admit the bar for that is pretty low.
I believe LastPass app encourages you to NOT do this.
This obviously doesn't excuse the implementation (it shouldn't of gotten past CR). Just pointing out the attack vector is not as severe as it seems (at least from the issue's title).
My password is 24 chars in length, includes digits and special characters and isn't made of dictionary words. Typing such a password on my laptop with a normal keyboard is fine, because I touch type, but typing it on my phone every time I need it is simply not doable. Using a PIN on the other hand is easy and should be fairly secure if the app is designed to fallback to the master password after X failures, preferably with hardware support.
So in all fairness, not only did they screw up a basic feature, but this also encourages people to use weaker master passwords.