Wouldn't, under positive g's, the top spar be compressed and the bottom one stretched? I heard the one that fails is usually the compressed one. Perpendicuar grain will therefore give the spar support so that it doesn't "cave in" (bad english?).

Upon further reasoning I think you guys are correct, I-beam would be weaker (but not that much) but it would also be lighter (less balsa and less glue) But the most important advantage of I-beam is that it looks awesome under transperent covering.