The rational behind having Trouble, Critical statuses is to inform proactively that the monitor is not behaving as expected and that it might go down soon if not looked into. Poll strategy is to ensure that the problem is real and not a one off spike in the monitor.
We send immediate alert if the monitor goes down which means there is a real problem to deal with.
We recommend you to first configure thresholds for trouble and critical and ensure that you receive alerts for them. This will help you to be on top of the problem before the monitor actually goes down.
I hope I answered the question. Please feel free to get back here for clarification.
Product Manager, Site24x7