It's okay to spend $X on customer acquisition if $X is less than the lifetime value of a customer (where X ends up being rather high for enterprise customers). But if it takes (pulling this number out of the air) two years to recoup that initial $X, then each customer is unprofitable for the first two years. And if you're a growth-minded SaaS firm, it's going to feel like a lot of customers are in those first two years: but once your initial batch of customers pay off their debts, so to speak, their profit can be invested back into customer acquisition -- it's not like your profits have to be funneled outside of the company, or that your growth has to be rampant and unchecked (with enterprise sales, you're more or less determining your own rate of expansion by the quality and quantity of your sales fleet). Acquisition begets acquisition.
Besides the fact that you need a cash reserve (either through your own savings or outside investment) and patience, I don't see what's particularly wrong with this strategy.