If the plasma membrane was permeable, but not selectively permeable, how would that affect the cell?

It would most likely kill the cell.

Explanation:

If a cell's membrane is fully permeable (not semi-permeable) then substances that are harmful to the cell are more than likely to enter it at one point or another, which can pretty much end up killing it.

In short, the semi-permeable membrane is there to ensure that only the useful stuff gets in and that the bad stuff stays out.