HI UnD3aD,
you are right that as mentioned in the datasheet of the pic there is weak pullup when you configured the port in MCLR mode, but if you see on this page 38 of the datasheet where the internal structure of the port is given then you can see that between the pullup and MCLR pin there is an inverter and pullup is also mosfet the operation of which is dependent on the MCLR pin
and the basic purpose of using an external pullup resistor in any microcontroller ckt is to assure proper high logic level because reset pin is critical in any circuit operation.
So I will recomend you to use a pullup resistor