I think the biggest single reason to learn OO Perl is because most CPAN modules use it. You can save yourself an enormous amount of development time by using the modules on CPAN, more than enough to justify learning OO Perl.
The best way to learn OO Perl is to buy Damian Conway's Object Oriented Perl. If you read that book and still don't know why you should write OO Perl code then you need more help than we can provide!