No, Inkscape doesn't have to. But, if the user is like me, and wants to maximize their time to be efficient, a program that does the same thing (vector drawing) but does export JPG, that program saves me time. That time savings seems to be something that is eluding some people.It's trivial to flatten a transparent PNG to produce a JPEG or other format using another application, if necessary. As such I see no particular reason why Inkscape needs a JPEG export option. These days if an application that is supposed to handle raster images can't load a PNG directly I would consider it to be broken. That goes doubly for a DTP program - what happens when you want a vector logo, exported as a bitmap, to be placed over another image? Good luck sorting that one out with a format that doesn't support transparency.
But don't forget to consider, the users at the other end may be using older systems and older software. Not everyone can afford, or sees any need, to have the latest and greatest of everything. When talking about any computer related topic, you should always consider the possibility someone in the group simply may not have the newer versions of X. And you have to work to that level.
But, if the program isn't supposed to support PNG, the program isn't broken, it simply doesn't have the feature you want. Just like I want Inkscape to export to JPG. That doesn't make Inkscape broken, it just doesn't do what I want/look for. Like a car that only has AM radio, but you want FM also.