I know. I recently acquired a board that uses a DS1307, and it's off noticeably after only one day. However, I have not observed the actual drift. It's possible that the frequency is fairly stable, so the long term stability might be okay if the frequency inaccuracy is calibrated away.
It is true that the clock time crystals (usually 32kHz) always have a turning point (where the tempco is most stable) around 25 degrees C. Other types may not be like that, but still perform as well as 10ppm for example, -20C to +75C for example.
Thus it is better to use a 32kHz crystal for clock time keeping.