Although a transistor is depicted as two diodes, in reality it is not as simple. For one, the base-emitter diode does not have a large current handling capacity due to the limit placed by the base current. Although you can tie the base and collector together to make the transistor act as a diode, it will work only for small signals or currents.
Another point to be considered is the transistor's maximum collector-emitter voltage (Vce), max reverse collector-base voltage and max reverse base-emitted voltage. These put constraints of how well the transistor can be used as a diode. For example, comparing an ordinary diode like 1N4007 with a transistor like BD239, we can see that the PIV of the diode is of the order of 200V (i.e., it can block reverse voltages upto about 200V peak), while the reverse base-emitter voltage (Vebo) of the BD239 is 5V.
