I don't think it will really have any impact to the overall Model Y experience, from an owners perspective. The part is lighter (something like 30%) compared to the two pieces currently being used. It should also have more rigidity to it since it doesn't have two pieces bonded/welded together, but how this will "improve" the model Y is really unknown. Overall production activities will be more efficient and somewhat faster, and in volume the overall cost to Tesla will be less. Do I think it should impact your decision to whether to wait for it or now, no, I don't. Like many have said, if you are always waiting for the next iteration so you don't feel like you are missing out, then what you will really be missing out on is owning a Tesla.
It wold be like saying people shouldn't buy the Model 3 because it isn't using the two piece rear structure like the Y, and instead should wait for the Model 3 to get the two pieces, or the 1 piece. Will it really change the overall experience of driving the model 3?