Why will happen if plant phloem does not contain sieve plates?
First of all, phloem sieve cells (opposing to xylem vessels) are live cells. In the phloem, a transport occurs not like in water pipes (with free movement of liquids) but through cells transport structures, particularly through plasmodesmata. As live cells, sieve elements need all standard structures, including cell wall, to sustain functionality.

Moreover, the phloem transport is (at least partially) an active process. I.e. to transport something you have to spend some energy. Today the most supported hypothesis of phloem transport is Pressure Flow Hypothesis . According to Wikipedia:


  While movement of water and minerals through the xylem is driven by
  negative pressures (tension) most of the time, movement through the
  phloem is driven by positive hydrostatic pressure. This process is
  termed translocation, and is accomplished by a process called phloem
  loading and unloading. Cells in a sugar source "load" a sieve-tube
  element by actively transporting solute molecules into it. This causes
  water to move into the sieve-tube element by osmosis, creating
  pressure that pushes the sap down the tube. In sugar sinks, cells
  actively transport solutes out of the sieve-tube elements, producing
  the exactly opposite effect. The gradient of sugar from source to sink
  causes pressure flow through the sieve tube toward the sink.


So, a plant needs live cells to support such mechanism.

Here we come to sieve plates. In plants, live cells interacts by means of plasmodesmata. In simple words, they are extensions of endoplasmatic reticulum which connect the cytoplasm of two cells and act as transport channels. And sive plates are simply places of highly hypertrophied plasmodesmata region.

So in conclusion:


Phloem transport is a partially active transport which occurs with
an involvement of cells transport structures like plasmodesmata.
The region of hyper-developed plasmodesmata on sieve elements wall
appears as sieve plate.
That is why an absence of such structure will not provide any gains in conductivity.


UPD. In response to Jim Young comment.
Damage protection is not a major role of sieve plates. Again, sieve plates are areas where plasmodesmata penetrate cell wall. Plasmodesmata act as the main vector of intercellular transport. So, the transport is the main purpose (or a reason) of sieve plates.  The damage protection is an additional property and not an aim of these structures. As to tyloses, yes it is a common mechanism of vertical transport prevention in xylem. But xylem vessels consist of dead cells. They are literally pipes. Therefore it is possible to cork up xylem vessels by simply inserting packs of fibre. BTW, that is why cavitation in vessels can cause embolization.  In contrast, phloem cells are live cells (with cytoplasm and EPR). They conduct water and sugars not by free flaw but with an involvement of intercellular mechanisms. That is why mechanical embolization is not a case for phloem cells.