California has decertified several models of voting machine because of security concerns. The problem is that there were several potential holes in the software that made them vulnerable. Some of it seems to be a no-brainer:
Um, yes. Wireless-programmable voting machines seem to be problematic. More of a bother to upgrade the software? Undoubtedly. But low cost isn't the only relevant factor.
The additional requirements she imposed included banning all modem or wireless connections to the machines to prevent them from being linked to an outside computer or the Internet. She also required a full manual count of all votes cast on Diebold or Sequoia machines to ensure accuracy.
It seems to me that regardless of your political affiliation or inclination, you have an interest in making sure the count is as accurate as possible, if only to ensure that those vile partisans on the other side (whichever side happens to be 'the other') don't rig it in their favor. (And a bias of 1 vote per precinct would be enough to throw an election. It doesn't take much.)
I'd rather be too careful than not careful enough, and security for something like this is difficult to get right. Must there be a zero error rate? No. There is an error rate, greater than zero, for every voting method. The most reliable method is hand-counting paper ballots, and even then, there are errors. This is why many states have laws requiring a recount if the outcome is within a certain narrow range, say 1% or less. If it's within the error bounds, you don't know for sure.
And if the machines were quietly set to disregard x% of the votes (those machines would be placed in precincts likely to vote heavily the 'wrong' way), or quietly record one or 2 extra votes one direction, no one would know. Thus, the need for caution.
If the totals are questionable, the legitimacy of the entire system comes into question. Unfortunately, the immortal wisdom of Boss Tweed may still apply: "As long as I'm counting the votes, what are you going to do about it?"