When to use a 2 pole circuit breaker over 1 pole
I see a few important questions to ask yourself.


What is the breaker's job?
How is the product connected to the mains?
What standards does the product need to comply with?


If the breaker's only job is to protect against overloading a single pole device is fine, but if (part of) the breaker's job is to protect against faults to earth then you should consider the possibility that your supposed "neutral" is not actually a neutral. 

For a single-phase plug in device the possibility of your supposed "neutral" not actually being a neutral is considerable. Even if your country's sockets are polarised people may take the device abroad, may connect it via a chain of adapters that don't maintain polarity or may connect it to a supply that doesn't even have a neutral.

So for a plug in device if your breaker needs to protect against faults to earth then it should be a double pole breaker.

A "single pole plus neutral" breaker that isolates both poles but only detects overcurrent on the live is useful when the breaker needs to protect against overloading and provide a means of isolation, but either does not need to protect against faults to earth or is used in a situation where applicable standards consider the possibility that neutral won't actually be a neutral to be negligable.