A planned hardfork, similar to certain softforks, leaves users with some reduction in security. It does not leave them defenseless. Consider the following:
1: Hard to be robbed on the basis of hashpower.
In reality the old chain will see mining all but stop, and blocks would be hours to days apart even if a couple percentage points of hashpower failed to switch over. Six confirmations would certainly take days. If the fork can be scheduled at the beginning of a difficulty period, the old chain would almost certainly not even ever make it to the next retargeting.
2: Hard to be robber on the basis of awareness.
Expect there to be fairly widespread coverage in the Bitcoin press, and as the fork draws near, maybe coverage in business and tech publications. Further, the alert keys will certainly be used, so node operators will get the message directly.
3: There still needs to be a targeted attack by a fraudster on an unaware node operator.
To fall victim, one needs to give up something of value to an attacker in exchange for Bitcoins (on the old chain). The typical uninitiated full-node user (probably a small subset anyway) is typically going to be buying bitcoin from a trusted source, and then saving or spending them, or perhaps gambling. They are not, typically, going to be providing a service or selling goods in exchange for Bitcoin unless they are at least somewhat aware of what is going on in the Bitcoin space. It's possible, of course, but we are talking about small numbers here of people who fit the above.
All three parts of the above would have to go perfectly wrong for someone to loose out. Someone somewhere will probably get scammed as a result of a hardfork. That stinks, and we should make reasonable efforts to help them avoid that fate. But at this point in Bitcoin's development, it is still in beta, it's still an economic experiment, and we can't allow the software to become hamstrung out of fear that some inattentive user might bungle their security. If they merely waited for 6 confirmations, as is the standard advice, they would be waiting for days. If that along doesn't give them a hint that something is wrong, it might still be too early days for them to be playing with Bitcoin for anything important.
I support a hardfork deployment that takes 80% of hashpower activate + a 4-month delay.