Interesting
I don't think it changes anything for miners if its optional. They will only participate if they can make enough money to pay costs and show a profit. They won't lock themselves in so easily as servicenodes, even for the right to vote.
There should be a way to lock miners in through some sort of hybrid collateral and Pow/PoS arrangement, more from a decentralisation point of view. There is an interesting proposals for a way to merge proof of work, with proof of stake and proof of activity
https://eprint.iacr.org/2014/452.pdfIf you can tie collateral to servicenodes to mining, then you have a way to increase decentralisation as buying lots of equipment is not enough, you also have to buy lots of SPR.
The decentralization idea seems good. I think we wouldn't want to take too many coins away from being eligible nodes, since the point is to support Bitcoin nodes.
My adjustment could be used as a temporary solution until nodes are self-sustained by the data market / DSDN. Once the nodes are making their own money, being paid by miners won't be necessary for the growth of Spreadcoin.
In the beginning though, I think ensuring that miners have a stake in Spreadcoin's success long term might be beneficial. This doesn't even necessarily mean that people will always vote for a reward. If a miner cared about Spreadcoin and Service Nodes were legitimately not serving any purpose then the correct thing to do would be to limit their funding. However, if they are serving a purpose and rogue miners can "attack" by voting zero for their own immediate gain, it could be detrimental to the network.
We haven't seen this in works yet, so it really doesn't matter now. I will bring it up again when we have Service Nodes.
I'm not sure what we would do if no miners wanted a vote (no miners want to put up a collateral). I'm assuming the mining reward would continue to be 100 percent for miners in this instance.