Besides the Pnei Yehoshua' objection that no sane farmer would plant a single row let alone a single seed and that how can you say a seed doesn't take up any space, in this case I think we can asked what the Rabbis accomplish with their tekanah. In any event I can plant arugot 6 tepahchim (which is essentially no difference than 5.7 tephachim) apart anyway. There would therefore be no need for the gezara to leave the corners empty and I would gain two rows of six tephachim north and south with three equal spaced seeds in the center.
Aryeh Shore
I am unsure why this suggestion is better. Assume, as the Mishnah apparently is, that the farmer (or probably someone who has a small garden) for whatever reason needs to plant five different types of Zera'im and four of them must be rows of Zera'im (he wants more than just one plant). The Mishnah's advice will work, while yours will not. Being that your suggestion only gives the ability to plant two full rows while leaving the other three Zera'im as mere seeds, the farmer will not be able to plant the maximum rows of crops in the space alloted.
Kol Tuv,
Yaakov Montrose