I think it defeats the purpose of the wood - for the back side to look good.

I don't want to sound too radical, but I don't see the point in paying for Lastucase wood with two ugly holes on the sides. 1mm is not such big of a deal and I don't think that slimmer necessarily means better. What I know is that people on Kickstarter have pledged their money for a great keyboard with good looks and I don't think that everyone would pleased if one part of this was to change (I wouldn't, even though I care more about the usefulness than the looks).

If I had a choice whether to buy the original piece or a 1mm slimmer one with an ugly back, I would definitely go for the original, thicker one.

EDIT: Well, I have pledged 20EUR more just for a better, professional look. If I knew magnets were going to be visible on the outside, I wouldn't have gone for the custom colored version and stayed at the 100EUR mark.