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This doesn't help much when the specific application you want to run is not available in your system because it doesn't ship Qt3 or Qt4 or Gtk1 or whatever (note that AFAIK none of these are shipped anymore in Debian sid) and it also doesn't help much if the application you are trying to run is using a library that, even though it exists in version N, there is already a version N+Y (Y=1, 2, whatever) that essentially provides the same functionality to the application but it is backwards incompatible while providing more functionality to the user that you'd like to use.

To make that last part more clear, imagine a theoretical "libopenfile" library that shows a file dialog, this library in version 1 exposes a function like "open_file(const char* path);" but in version 2 they improved the GUI a bit (e.g. making the dialog make better use of screen real estate, added a create folder button, etc) and changed the function name to "lof_select_file(const char* path);" because arbitrary tidyness reasons and in version 3 they changed the signature to "lof_select_file(const char* path, unsigned flags);" and added the ability to load plugins that could provide more actions for each file in the displayed file list, show overlay icons, etc.

As things are right now, a modern Linux system would need to have all three versions of the library installed - assuming the distribution cared about backwards compatibility to provide all of them instead of the latest and perhaps the previous version - meaning there would be three different libraries providing essentially the same functionality in three slightly different ways. Moreover, and also very important, version 1 applications do not get to use any improvements introduced in version 2 and neither version 1 nor version 2 can use the plugins introduced in version 3 - even though, again, all they want is to show a dialog to choose a file.

The way that would have been done right is for the version 2 of the library to still export the "open_file" function but simply make it call "lof_select_file" and version 3 make a "lof_select_file2" (or "_ex" or whatever, if the reason to not do that is because you dislike the naming, you are part of the problem) that accepts flags and have "lof_select_file" call it with a default set of flags. Then everything would work, from version 1 to version 3 and all applications would get the functionality introduced in subsequent versions (and as a sidenote, no, i wouldn't remove open_file from the header files to encourage using the renamed function since backwards compatibility in source form is very important too - not everyone who gets to compile a piece of code is the developer of that code, or even a developer).

Now the above is a bit of a strawman, but it is exactly the situation you get with Gtk1, Gtk2, Gtk3, Gtk4, Qt1, Qt2, Qt3, Qt4, Qt5, SDL1, SDl2, etc and pretty much most of libraries that you find on a desktop system (notable and thankful - so far - exceptions, being the X11 libraries and... Motif, although that isn't installed by default anywhere so it isn't really something you can rely on).