Example: ticket #52 has keywords set to "test ticket blah". If I use the BatchModifyPlugin capability to set the keywords field for ticket #52, the existing keywords values were removed and the new ones set.
Now, it could just be a matter of differing expectations, but I think in this particular case the plugin should check the value of the keywords field and decide what to do accordingly: any text input should be appended to the existing keywords value (with requisite space inserted); if and only if the input value is empty would the existing keyword value be removed.
Just my $0.02...
Note: I'm using BatchModifyPlugin on Trac 0.11