Some domains are truly worth $1 million. If the guy has plenty of additional money to invest, why would it matter? A really great domain can increase your chances of success.
It's also the only domain of those four that had significant inherent value. They could have easily resold Mint.com for a profit. Buying it early gave them the benefits of the great domain and lowered the price they paid. I think what Mint did was brilliant.
The only reason I can think of not to buy a $1M asset for $1M is if it eats into your runway. If it doesn't then you're better off if you can.