You can still buy it once when the game is actually completed / for retail :3. There will likely be an early access period when this is possible. Doing this preemptively will damage a developer's ability to negotiate with retail platforms (Steam for example has requirements about where the game is sold and for what price).
Helping indie developers out at this stage of development is completely up to you, but it actually helps. Pure Onyx will have a budget roughly 1/200 that of EA titles, so it's apples and oranges.