Beside the fact that it takes much time to complete one bitcoin from a signature campaign someone should also bear one more thing in mind that signature campaigns are not for always, so no one should take them as a full time income and just rely on them as they might get finished tomorrow and then you will be earning nothing if it was your only source of income.
It's not a big amount, but for an extra income it's fine. Before BTC, there was a lot of PTC sites, and many people used that, but very few people made some money because they had lots of referrals, and many didn't make anything, because they didn't have referrals.
Now with BTCs everybody can make a decent profit here on forum without the needed to have referrals. It's a good point, everything here depends only about you. You don't need others clicking for you to make comission. That is why many fellows here devinize signature campaigns.