If a farmer regularly deletes unpopular data, their node will be asked to redownload a copy of that data frequently, increasing their bandwidth costs. If the node refuses any request to download a copy of data, or pretends to download and not actually bother, the storage the network believes it to share is reduced. Your allocation on other people’s machines is therefore also reduced, and your costs for keeping your data on other people’s machines rise and therefore your surplus drops.
The handy thing about SHA512 content addressable data and an XOR chunk map is that chunks really are allocated as close to randomly as is possible. So long as you supply a reasonable amount of storage to the network, your income will tend very closely to match the network average. And that network average is the optimum amount, it’s as good as you can get for any sustained period if you supply a reasonable amount of storage.
MaidSafe spent a ton of time thinking of every conceivable way one could game the system and made sure you lose out from it. Sans bugs, of course.
Niall