From a practical standpoint, it makes it more difficult for me to trust that it will be maintained in the long-term, or that I can extend its functionality if I see a need.
Ideologically, I'm somewhat uncomfortable using duplicacy when fully-free alternatives exist. I'm not a free software purist by any definition (I use steam. I have a Netflix subscription. My android has google apps on it.), but this is an area where compelling free software solutions do exist.
The license also keeps it from being packaged in most Linux distributions, which makes it a nuisance.