That's true, but:
1) Inputs can accept '51 output, no buffer used. Your device works. Loss 0, gain: buffer price (some $0.15)
2) Inputs can accept '51 output, you use the buffer though. Device works. Loss 0, gain 0.
3) Inputs can't accept '51 output, you use buffer. Just like above.
4) Inputs can't accept '51 output, you don't use a buffer. It burns the pull-ups in '51, gain 0, loss - value of the '51, some $1-10.
5) The device is broken and outputs way too much current (and you don't know about it), you attach it through the buffer. Buffer gets burnt, loss $0.15, gain: The '51 chip survives.
6) The device is broken, you attach it directly. Loss: The '51 chip.

As you see, the buffer is kind of insurance policy, either you use your knowledge and learn whether you don't need it, or you use your luck and hope you don't need it, or you just use it and you're on the safe side.