You can freely use it as you please. If you have a previous version that works for you, feel free to use it to its fully extent. We are glad many Roundcubers do find value in these plugins
If you wish to stay current and if you would like to always have the latest code with bug fixes and the such, plus a way to allow end-users to manage plugins per-user level, maybe Plugin Manager is for you (optional). You are also free to use these plugins in as many servers as you own without having to pay for them separately.
From what I've seen (and I could be wrong), people tend to automatically associate prices with licensing, forgetting that those behind these plugins (as well as Roundcube Webmail development) are human beings just like you and me. I will strongly encourage everyone to support what we are doing by purchasing a plugin download and to also contribute with donations to Roundcube Webmail. There are users who can't afford to donate or pay for anything but that's also the vast minority. The reason why I dare so say this is because anyone out there looking to host their own webmail solution is incurring in some expenses one way or another. Either a VPS, a hosting provider or any other type of private infrastructure to start with, and the list could go and go. They then turn to Roundcube Webmail and never take the time to click the donate button even once and contribute with those who really put time and efforts into this sexy webmail. But hey! next month they have to pay for hosting or the VPS again, you know. You will also find those who will stand behind the curtains and bring up the "Open Source" philosophy and its history to justify their actions or the lack of any contribution therefor and that has become a norm for many.
At the end, your contribution motivates Roundcube Developers, our team and helps others in the process (
http://www.beat-richner.ch). Do what you believe is right.