Let's say there's a document which contains 30 or so revenue stamps which are not recorded anywhere else. Retaining the document intact means that only one person can have those 30 stamps, but soaking them off allows multiple collectors to have an example of that stamp in their collections. It would probably make more sense financially to soak them off as well, since it's likely that you'll earn more money selling 30 separate single stamps than all the stamps at once on a document.
Of course, it's a pity to see documents with historic value (and which might be unique like my example above) be destroyed in order to remove the stamps - and I would definitely not do it myself - but there are cases where destroying one document allows more collectors to own a stamp, and this is arguably more beneficial to the hobby.
The situation is similar to breaking up a sheet of stamps. If the only sheet of Inverted Jennies which was purchased intact from the post office in 1918 had been kept as such, today all copies of that stamp would be owned by a single collector or a museum, and most probably it wouldn't be as well-known and iconic as it is today.