Say that you've calculated that over, say, a 1-year term (long time in the Bitcoin mining world), investment and ongoing costs combined means you should be getting about $1000/month. That's $1000, not some Bitcoin amount, because the warehouse wants $, your landlord wants $, your baker wants $, and you purchased the equipment in $.
You may or may not get that back from mining and selling the mined Bitcoin. So you offer shares, 10% for $150. 10 people buy into it, because the $150 still appears to be a good deal compared to competitors / buying and running their own hardware, especially since you bought in bulk and thus were able to get better pricing. Now you get $1500/month, no matter what the exchange rate is doing.
At least, that's how I imagine it works out for farms - if they can get enough BTC themselves, believe that they will continue to get enough from it, and they would make less from selling shares... then obviously selling shares makes no sense whatsoever.
The argument is similar for hardware manufacturers, of course. Unless they're staunch believes of the Bitcoin system and want to see it grow in stability, then the only reason to sell the hardware rather than locating and running it themselves is because they much prefer the guaranteed (pre)payment over the 'if all goes well' BTC returns.
People with Block Erupters should adopt selling mining shares, really
