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.
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.

You want JPEG export. I know others that would like Photoshop PSD files. Yet others will want TIFF, or BMP or other esoteric formats. Should Inkscape support them all, or would it be better to export one or two formats and allow the user to use another application to convert to whatever they want?

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.

Where do you draw the line, though? How old should we cater for. What if someone's still using software that pre-dates JPEG? Hmmm... perhaps Inkscape should just export in one or two open formats that can easily be converted to others - even pre-JPEG ones - using other software.

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.

If a DTP program doesn't support a bitmap format with alpha channels then I *do* consider it broken as it prevents you from doing a lot of basic things, like putting a logo over an image. It doesn't have to be PNG, but in practice that's pretty much the only widespread open format that does the job.

Inkscape not exporting JPEG doesn't prevent you doing anything, it just adds an extra external step if you absolutely must have a JPEG.

Therefore I consider one case to be broken, and the other not. One stops you performing the basic tasks the program is there for, the other doesn't. Inkscape exports in the only widely used open raster format that supports alpha transparency and saves losslessly; from that high-fidelity original it's down to the user to accept the trade-offs of converting to something else, and to pick their preferred tool for doing so.