Sending a constant stream of cheap units into battle is a completely degenerate strategy and there seems to be no counter to it.
Just because a lone weak unit can sometimes do larger than expected damage, doesn't mean that that would be an effective long term strategy.
Even if that did prove to be effective (has anyone actually recommended spamming cheap units and spreading out as thin as possible?), it could easily be countered by using cheap units along the border of someone doing that, or by advancing your own cheap units.
