I agree with RentFree's answer, but another part of the answer is the cost of capital. ButterflyLabs did not have the cash on hand to build its ASIC design or to manufacture all of the rigs. That's why they had their pre-order process: collecting the money up front was the only way they could afford to build the rigs. (Once they had built them, of course, they could have defaulted on their promises and used the units to mine for themselves instead of delivering them. But that would have had long term consequences to their reputation. I don't think the question was really asking if they should commit fraud, just whether they should keep their design private.)
The question does raise the question of whether it would be possible for a different company other than BFL (say a private equity firm with deep pockets) might fund their own ASIC mining rig and not release it to the public. I wouldn't say it's impossible, but it does have several factors against it. Firstly, RentFree's answer: that a private ASIC design strategy would have a lot of inherent risk. But there's also the issue that much of the cost of building an ASIC rig is in the design of the chip. Once our hypothetic private equity firm has spent the money to design the chip, the margin on individual rigs would be fairly high. So, at that point, there would be little incentive for them to refuse to sell the rigs to others. (Even if it raised the mining difficulty for their private rigs.)
In short, while there is nothing to say the ButterflyLabs (or other ASIC manufacturers) might not use some of their rigs (or test rigs) for mining, the business model of these types of companies wouldn't work very well if they didn't sell the majority of their machines: the upfront capital costs are too high and the incremental profit margin is too tempting.