>
> I only meant that double spends minutes apart are possible and that by then
> the sole use of a monitor is too late even if it will tell you.
>

As long as miners stick to Satoshi's first seen rule, which is the default,
it's useful:

https://bitcointalk.org/index.php?topic=423.msg3819#msg3819

(this is the famous "snack machine" thread from 2010)

If they decide to change to something like highest-fee-always-wins, then
they (again) centralise things by forcing all instant transactions to pay
GreenAddress and its competitors money - much though I like your product
Lawrence, let's hope they don't collectively lemming us all off a cliff by
doing that ;)