Because it is the only service which let's you run complex monitoring capabilities available in JMeter scripts.
Yes. You can use any components which comes with JMeter 2.9 without any limitations.
We want a faster internet experience. We believe that by providing some performance monitoring capabilities, for free, for those who want to invest in it, will help to achieve this target.
For now you can only use a single script at a given time. If you want to change/replace the script, you need to first delete your existing one and only then you will be allowed to upload another script. Don't worry - your old monitoring data will remain available for you on the dashboard, we will not throw it away. We might allow more scripts in the future.