An interesting idea, as onions hate competition.
However I can think of a couple of immediate drawbacks, watering being the main one. To bulb up to a nice size, onions need a surprising amount of water and getting it to them through plastic would be a headache. Using weed supprecent fabric would be better but would still suffer from problem 2 below.
The other problem comes when they start to rise out of the soil towards the end of the season. If the hole is too small it will lift the plastic and be trapped underneath so it cannot dry out. If it's too large then you will be weeding anyway. Likewise when they sprout, if the hole is too small and they miss, you will lose the plant.
As onions swell throughout the season, you will never be able to cut the plastic to the right size all the time, which means you will still end up weeding, and take time recutting holes.
It's probably quicker to hoe.