So Trello is not currently profitably? "Fog Creek is profitable and could afford to fund Trello development to profitability." Given its growth rate and presumably low sales / marketing expenses, it would seem to be easy for them to be profitable.
I am no longer a Fog Creek employee (I left to join an education startup a bit ago), so this is not an official opinion, but anyway:
Joel wants Trello to grow a lot faster than Fog Creek could bootstrap it. In my personal opinion, a big reason why Copilot and Kiln never quite made it was that we didn't have the developer resources to dominate the market when we were in a good position to do so. Because we insisted on bootstrapping, we necessarily had very small teams, meaning that competitors, who were willing to go into debt to have larger teams, were able to come from behind and surpass us in both marketing and features. In other words, while both products are successful and profitable, they likely could've been a lot more successful and profitable if Fog Creek had thrown a lot more resources onto them back at the beginning.
What you're seeing with Trello is an attempt to avoid that happening again. Trello is currently in a strong leadership position; taking VC now to secure that will pay for itself later. While Trello could likely be profitable very quickly if Fog Creek wanted it to be, doing so would require a very small team bootstrapping itself again, meaning that Fog Creek could repeat the same mistake a third time. And that's why you're seeing this.
I don't agree with what you wrote at the end. It's not a "mistake" to bootstrap and have a much higher probability of success with a lower possible return. I know that's probably not what you meant, because you wrote earlier that we could have been "more successful", but it also ignores what would have been a much higher risk and loss of ownership in your assumption.
Joel and I are naturally conservative in nature and having succeeded at Fog Creek with what we set out to do (building a successful company where devs wanted to work) allowed us to swing for the fences with Trello and Stack Exchange. The risk is much higher. The reward is bigger, but we own a lot less of those VC funded companies.