[aes256]

A few obvious drawbacks without even reading into the subject:

1) The additional sense of security may lead people carrying pepper spray to take more risks, to travel through more dangerous places, etc. (risk compensation, a la bicycle helmets)

2) Few people who carry pepper spray have received appropriate training in its use, and even those who are appropriately trained will take a not-insignificant amount of time to retrieve and effectively deploy it;

3) The person using pepper spray is almost certainly going to fall victim to the pepper spray themselves, hindering their escape from the attacker;

4) The use of pepper spray is likely to anger an attacker, leading to an escalation in violence; even if the attacker has difficulty seeing as a result of the spray, they may still inflict an awful lot of harm on their victim;

5) In the worst case scenario, in fumbling to retrieve and deploy pepper spray, it may be snatched by the attacker and used against the victim.

[icey]

I don't know anyone who carries pepper spray that's become less risk averse because they carry it. Isn't that kind of like saying carrying condoms makes you more promiscuous?

The question is pepper spray vs. no pepper spray, I think for the rest of your drawbacks it's pretty clear that having a deterrent available is an improvement over not having one.

There's quite a lot of literature about this, and I guess you could ask any cop what they think about it too.

[aes256]

Risk compensation is observed all over, and is often subconscious.

You carry pepper spray, you figure you've got things covered if you happen to be attacked, so you're prepared to accept a slightly increased risk of being attacked as a trade-off for other benefits (e.g. saving money on a taxi home, or time by taking a shortcut down an alleyway)