SVG rendered via an <img> tag has restrictions in browsers, for example Firefox will not allow the SVG to reference an external image file such as a jpg. Once again, you put the raw SVG file in the browser and it does not render...
Obviously I miss any SVG 1.2 feature which Inkscape lets me easily use. Compositing for example. User has "SVG" output from Inkscape, user puts it on an "SVG" hosting service, it does not render in the browser = unhappy user. Users don't care less about what version of SVG they are using.
Stripping browser-incompatible elements could be useful, for example, if I place my content inside an SVG 1.2 <page> then the browser will render nothing at all, where as if the server stripped out the <page> element first, I'd be able to see my content. Obviously this applies only to a single page tag. Once again, you put the raw SVG file in the browser and it does not render...
Bug-workarounds are the primary reason for wanting to manipulate the elements though, there is a Safari bug which can cause it to ignore <defs> but work if you switch out <defs> for <g>. Once again, you put the raw SVG file in the browser and it does not render...