The more people playing it will only equate to more word of mouth and a farther reach into it's possible consumer base. Keeping in mind that the guys at Mojang don't do much advertising, if my friends / family talk about minecraft with other people, and that gets them interested and they buy the game isntead of borrowing my account (I know of at least three friends who have bought the game because I began posting shots of it to Facebook), that only equates to more money for Mojang when the game is actually released.
This is also along the lines of what Notch has previously stated on his tumbler account: How Piracy Works
And an article related to this containing quotes from Notch himself during a press-release: Press-Release
And all of that being said, depending on how good the final release of Minecraft is, I'll probably buy it when it hits release.