Im not suggesting that I be guaranteed to store a copy of the data for the user I set as preferred, only that if I do the contract will be set at a discount. Imagine a cancer charity signs up for Sia, 25% of the available storage hosts may be prepared to get a reduced return if they host any of the data. Im assuming the charity would have to make 6 contracts per set of data (one with each user storing the data) and would therefore have a 25% chance of getting a lower rate than on the open Sia market.
If Im way off track, no worries, just a suggestion.
I think its an interesting proposition: be able to set a discount for organizations your support. It would require some changes to the protocol for sure. The simplest 'fix' would be to change the hosting charge on the fly as preferred IPs connect. But this would mess up the host selection algorithm; when uploading hosts are heavily biased by cost, so cheaper hosts are preferred before more expensive hosts.
I guess that as we see Sia being developed, there will be more options to configure and use Sia per-user-preferences. Currently, we're locked into a bunch of defaults, but I'm sure adjustable parameters will be opened up for customization gradually in the future. Whether we see this is up to the devs, but I like the idea.
So lets file it until a time when there is less heavy lifting for the devs, but I could imagine giving users even a perception of control over what they are storing would be popular.