Let me give you the exact application, I was thinking about use all 10K connected to P2 of an at89c52 to make the ladder,...

Hallo Gabriel,

keep in mind, that at P2 strong pull-ups are only activated for two oscillator periods. Rest of time only weak pull-ups are activated, which represent a rather high impedance. So, performance of P2 outputs, when not used as address ports for adressing external memory but only reflecting content of SFR, is far away from behaviour of classical 74HCMOS outputs.
So, it seems to me not to be a good idea, at least not with rather low R-2R resistors like you planned to use.
In an old application from 1985 '8048' was used with a R-2R ladder consisting of 180k and 360k resistors. Additionally port outputs were pulled-up with 12k resistors. An LM358 was following to R-2R ladder. Output of this OPamp was fed by a 470R resistor to input of CD4052. At outputs of CD4052 22nF hold capacitors were connected to ground. Additional port lines fabricated decoding.

Now you see what's necessary: You must drastically increase resistor value of R-2R ladder and add some adequate pull-up, or just use an additional 74HCMOS buffer.

Please keep in mind, that in 1985 it was much harder and expensiver to realize multi channel DA-conversion than today. So, I tend to agree with Steve and Michael or even Andy.
But if precision is not so important and costs is crucial, why not use a simple R-2R ladder? To be true, cost advantage of discrete solution is becoming smaller and smaller, nowadays...

Kai