That depends on the nature of the data. With a DAQ card, you'll be able to capture/generate raw data directly with the PC whereas the RS232 port in your computer is designed to communicate with other RS232 devices via async serial.
That said, there are some (what I consider "low quality") DAQ applications that people have used the RS232 port as a 'free' DAQ interface. If the voltages being measured/generated are compatible with RS232 (+-0.3V to +-25V) then the modem signals can be used for either input or output signals. For example, if I had a voltage that went from -5V to +5V and I wanted to know when the voltage changed, I could use a RS232 input pin (DCD for example) and then look for a state change of DCD.
The reason I consider this "low quali ty" is that you have very limited control over the output voltages while giving up tight timing. You also have a limited number of I/O (DTE is 3 TX, 5 RX, but 1 of each is for data), and you run the risk of destroying the RS232 port and/or motherboard in your computer if you exceed the maximum voltages (there is no protection - unless you use an isolated board, which NI does offer).