Disadvantage: You might not make a profit, in the long run, and the time and money you spent will be lost.
Advantage: You learned that it does not work for your site, thus causing you to improve your site for conversions.
Disadvantage: You could have spent your time and money on building other incoming traffic.
Disadvantage: If you are not motivated to improve your site, to improve your ROI, you may lose some confidence.
Advantage: You found out that your site actually does really well, using PPC, and you keep pouring money into it, 'cause you are making a profit from it.