Does Adblock Plus block scripts and phishing attempts from 3rd parties on websites that might target your wallets/exchange accounts?
Nope, what you want is "NoScript"
Also use Adblock Edge, they dont have "allowed ads"
I know the most secure thing would be to disable JS altogether. But is this a middle ground that still lets you use the functionality of most sites?
Thats what NoScript does. You can enable each JavaScript individually either just for the running session or in general by domain.
I hear that a lot of people are getting their blockchain.info and BTC-e accounts hacked and it makes be suspisious of third party attacks being inserted on sites via an ad campaign.
Good idea or useless?
Possible. I read somewhere that drive by infections (via ads or other online sources) ist the most common way to get infected nowadays. Not sure if thats true or not.