As long as you keep the wallet open and unlock to stake then you are one of the stakeholders. The point is to keep the network as secure as possible and only people staking can make that happen. Along with gaining more BTCD, the dividend is pretty much an incentive since so much of SuperNET will be riding on BTCD. There is also mention of checking how long each user stakes to determine fairness of dividend so that opening the wallet once a month for a few hours doesn't gain the same benefit as someone that always keeps theirs' open. Basically, you are rewarded twice over for keeping the wallet open and staking.