For kicks, lets say I purchase these miners with 15 BTC at a $100 USD conversion rate. Is it likely that I will mine 15 BTC with these erupters? Not likely. But, since I effectively purchased them with fiat, recouping 15 btc is no longer a concern. I simply need to recoup $1500 in mined BTC to achieve 100% ROI in the scenario. If the value of BTC increases and continues to do so significantly, achieving 100% ROI becomes more and more likely.
Of course the profitability question is still whether it will mine 15 BTC or not. The original 15BTC are irrelevant, whether you bought 15BTC after buying the miner makes no difference to the profiability of the miner. You effectively bought a miner for 15 BTC. You could have bought coins instead and had 30 BTC.
case a) buy 15BTC + 15 original BTC = 30BTC.
case b) buy miner, buy 15 BTC with fiat (e.g 1500)
The only way case b is more profitable than case a) is if (mined coins) - (running costs) + (resale price of miner) > 15 BTC
regarding the resale you're relying on someone else buying it from you at a price where the miner will not be profitable to them.
If you can't do this basic reasoning maybe it's better to stay away from ASICs
I'm assuming the price of BTC, in the long run, will rise. As I think that unless BTC fails completely this is inevitable. To hedge against falling BTC prices, you could buy in gradually, buying some every month instead of all up front - you don't need to get miners involved to achieve this.