It's a religious debate. Some would argue that it's better to have a small card fail and only lose part of the days shooting. I would argue it's better not to have to swap cards because you increase the chance of losing a half a day's shooting.
Realistically card failure due to actual failure (rather than breaking it, accidentally formatting it etc) is rare. The risk increases with the age of the card. Go with an 8gb now, you can always buy a second 8gb card when they cost nothing in a few years.
And I agree with what you say, but I come from a long career in IT, where you learn to expect failure and prepare for it.
Most all of us have likely met "Murphy" and his law when we were not expecting it. Always seems to happen at the worst possible time.
I just tend to go smaller, as I have seen a couple of cards fail, and in one case someone formatted a card when they shouldn't have. For me, smaller 4GB cards just lower exposure to losing photos, and I can store two or three full cards in my bag almost as easily as I can keep one until I get back home to offload them.
I realize that a card failure is rare, but I just don't want it to happen to me, nor do I want to lose much data if it does.
But consider me a conservative old fart - I can also remember when a pc came with less than 10-12GB of hard drive storage, and was considered to be huge at the time.
And not so long ago, an IBM 370 mainframe computer, one that would host the online application for all the teller's terminals all over town, ran with 1MB of memory. The computer room was full of rows of disk drives, all combined having a total storage capacity of less than the PC we each use to read this posting.
And nowadays, our wristwatches, cellphones and appliances have more processing power than did those 1970's mainframe computers.