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 ;)