Hi Ramiro,
you don't have to use a client-side certificate if you don't want to - but you can - it depends on the server you are talking to.
For example, if I'm using NetWebClient (which is based on NetSimple) I can access a web site without using a client-side certificate. (it uses the ca_root.pem file, but that's something different.)
However when I talk to a payment processor, and they've given me a client-side certificate (and key), then I do need to use that in order to connect to their server.
So, I guess the answer to your question is;
>> To use Netsimple with SSL is it mandatory to use certificate and key on client-side?
It depends on what the server you are talking to requires.
cheers
Bruce