It is interesting to note that the first patch to enable a full-screen mode in ePubs (by metazoa of the Russian forum the-ebook.org) was made back around firmware version 2.6.1 or earlier.
It wasn't until later, in firmware version 3.3.0, that Kobo added their undocumented full-screen option. It was quite an improvement on the original patched version because it worked with KePubs too (albeit imperfectly.)
Later on, MR members found a way to work-around the problems that affected KePubs in full-screen mode, first by editing sideloaded KePubs (no patch required), and then by altering the device's built-in KePub stylesheet with a patch. These are not perfect solutions, but they make reading KePubs in full-screen mode quite workable. (I use full-screen mode exclusively for reading both ePubs and KePubs now.)
I expect the next step will be that Kobo finds better solutions to the work-arounds and enables the full-screen option by default. My guess is that they haven't done this yet because the problems that affect KePubs are not the fault of Kobo's full-screen code itself, but are actually bugs buried deeper in the KEPUB reader (possibly in third-party librarys that the KEPUB reader uses) which full-screen mode merely makes more noticable. (one of the problems is visible on a stock reader if you set the margin slider to minimum while reading a KePub in normal reading mode. Activating full-screen mode just makes this existing bug visible at other margin settings.)
A similar sequence of events occured with the font sizing:
Originally the Kobo firmware had 24 font sizes ranging from size 12-58, and there were some very large gaps in the sizes that could be selected at the middle of the range. For example there was only one size selectable between 32 and 40.
A patch was created (metazoa again) that allowed these gaps to be filled in and so made possible finer adjustment of the font size in the middle of the range.
Later, in firmware 3.12.0, Kobo increased the number of font sizes to 40, but still didn't fill in the large gaps.
Later again, in firmware 3.13.1, Kobo increased the number of font sizes again (it now varies by device, the latest devices have 48 sizes) and partially filled in the large gaps, now there were 3 sizes selectable between 32 and 40
However the font sizes patches continue to be used, because some people want even finer adjustment than Kobo have provided for in the sizes at the middle of the range they most often read, and don't mind giving up some of the huge sizes at the top of the range that they never use anyway.
There are also a number of patches that are no longer needed at all because Kobo have completely fixed the problem that the patch was created to solve. In some cases the way that Kobo fixed the problem was identical to the method employed by the patch. (For example,
this one.) Those who patched were able to enjoy the fixes for a year or more before Kobo got around to making them part of the official firmware.
So the existence of patches hasn't stopped Kobo from continuing to improve the features that have been patched.