Well there are different ways to approach this.
I agree with Sides that it is naive to buy a game before it is finally released and before you can see if you are going to enjoy it.
Also, when you hire people (for example construction workers), it is not a good idea to pay the whole amount before they finish the job. They may be tempted to do the job fast and with low quality, because after all they will not get more for doing it well. You should pay them some initial amount, but wait with the final payment until you are satisfied with the execution.
This may or may not apply to game development, but some ways of human thinking are universal.
On the other hand, if someone wants to spend money on entertainment, and buying this game makes him happy, then he has sucessfully bought his entertainment.
As for LegionTD, I don't remember it from WC3, although I have probably played it a few times. The impressive number of sales they got already indicates it must have been quite popular, or maybe thats mostly due to the SC2 and DOTA versions.