The key reason the ASIC miner companies can't keep the machines for them selves is they rely on pre-order capital to manufacture the units, arguably there are "delays" on these units getting to the customer. If you was to have 50 units "testing" for a number of days before shipping you could as a miner merchant make a fair few Bitcoins, note that I say could as this has not been proven and these units do make a big difference to the network once connected especially the 600gh/s units on pre order now.
Hehe thats really not a neutral view on the subject. The path from bitcoin to real money is short, although you dont know how much money you will get from day to day.
You may argue that selling these machines gives you a more predictable income, but that hardly holds water as the demand surely will follow exchange rate.
The only valid argument left seems to be the timeline of wich the money comes in. If they sell a lot of machines early, they certainly will get the money back slightly faster, so for the buyer, its a highly speculative trade.