Paper votes are subject to impersonation, for example, especially if voter turnout is low. During canvassing for the recent UK General Election for example, I became aware of people who were not voting due to absence (and hadn't secured a postal vote). It would have been simple to use those votes if I was so inclined.
The only solutions are transparent voting systems (if electronic, software and hardware must be publicly documented so that flaws are found and fixed - yes, I user Firefox!), plus independent audit trails (say, issue each voter with a receipt that can be checked against the voting record, if they agree).
The inconvenience of paper voting (many hundreds of people couldn't vote in the UK due to various issues related to this, and unexpected voter turnout) will push us towards electronic, probably internet voting whether we like it or not. The real question is not are these systems acceptably fraud resistant, but how to make them so.