If someone already implemented an XML parser for you, SVG is easier to implement than TVG (if you are trying to extract the same type of content out of SVG as out of TVG).
>if you are trying to extract the same type of content out of SVG as out of TVG
But then you haven't implemented SVG. You can't say "we support SVG", and you lack a good way to communicate what users can expect from your implementation. It's much easier to communicate clearly when you can implement a whole standard rather than a haphazard subset of one.
Also, parsing overhead is such a tiny fraction of the overall effort (in either case) it doesn't really mean anything.
