The use I have for the data are primarily for accurate display of the sky at any given moment which the post 1970 data does very well. Astronomers avoid using zones generally and use UTC or GMT/LMT for past events but there are some programs such as Starry Night and Skymap (any my software) that use the PC's clock and a zone offset (which can be set to past dates researching such phenomena as the timing and accuracy of occultations with select planetary perturbation terms, etc). Astrologers (not astronomers) have been the primary source for this older information and anyone who wants to display the sky with only the knowledge of what time is on their watch would be interested in this historical data. While the time zone information of the past (particularly the first half of the 20th century) is largely suspect (and really anyone who has done even modest research can tell this), it is important to anyone with this interest. Since the historical data doesn't change much, I maintain a separate database for this info (with warnings) since the tzdb can't promise that it will always be there and many have no interest in this type of use.
One might want to keep in mind that there are many who have written access to the tzdb in other languages. Here is one example:
To some extent the historical usage accuracy issue is offset by the fact that the particular use I describe above has worn grooves into the more densely populated areas which has a tendency to be more accurate than outlying areas.