It seems to be very close to the surface of the spar, and on some examples evidently touches a little. Under flight conditions, I suppose it's possible for the fabric to touch the spar when the wing flexes, etc. Anyway, I think it's pretty standard practice to add finish tape sometimes when the clearances between fabric and structure are real small, and the possibility of contact under certain conditions exists. In this instance I kind of like the tape there because of the relatively large unsupported areas and the fact that the wings do flex. That's just my opinion.