Whether broadly or narrowly conceived, symbolic violence isn't actual violence, and whatever the relation between them, it is useful to know the difference. Political and social systems are backed by violence, in the last resort and sometimes day to day, but this doesn't mean that all the negative features of such systems are - themselves - violence. We need some linguistic means of making distinctions, otherwise we should not be able to talk clearly. Or, put differently: there are other evils in the world than violence.
I haven't yet read Zizek's book, but I agree heartily with Norm's view and with Nigel's critiques. (Zizek, I think, is sometimes interesting and thought-provoking and sometimes merely maddening.)
Violence interests me a bit, and I've become wary of expanding the concept beyond the bounds of precision, i.e., beyond meaning something like 'the infliction of non-consensual physical harm'.
There are, as Norm says, many 'evils' in the world other than violence, and the left has long had a good (though evolving) vocabulary to describe them, such as 'discrimination', 'exploitation', 'social exclusion', 'injustice', or 'poverty'.
I doubt that any analytical gain is made by referring to any of these things as 'violence' (whether 'systemic' or 'symbolic' or whatever).
Those are good words for bad things: more precise and (at least potentially) better focused on what's actually going on in each case. Replacing them with 'violence' sounds more dramatic but seems to muddy the matter. (Even within the realm of physical violence, after all, the term can be too broad, including, as it does, everything from school-yard bullying to mass-murder.)
Whence comes this violence-creep? One reason may be that the post-modern academic obsession with text to the exclusion of reality might make this kind of free-association easier. Another, I think, may have to do with a loss of confidence by some on the left about the validity of their traditional political vocabulary. This may arise either because they feel uncomfortable with it themselves or out of a more tactical consideration that population at large may no longer be moved by appeals in such a vocabulary.
Some may think, then, that 'violence'--being widely seen as a Bad Thing--can be taken over and used for any purpose.
I doubt that very much.