If your computer has a ride time/total time readout with seconds, you can calculate your cadence pretty easily by counting the number of downstrokes on one side within a 6-second interval, then add a zero to that (e.g. 10 downstrokes = 100 rpm).

For the few times I've actually wanted to know my cadence, this has been sufficient for me. After a while, I have gotten the "feel" of the right cadence for the job, and I just go on that.

That having been said: the difference in cost of computers with cadence as compared to those without is certainly low, so if a few $$$ isn't an issue, might as well just get it. But if you are on a tight budget you can probably do without it.