People can literally open programs on your PC if you join a game, basically giving them access to your entire computer. In this case, the calculator was opened just for demonstration purposes, but what if it wasn't just the calculator app? Exploiters could..
- infect your computer with viruses
- get access to your browser passwords
- get access to your files
- literally anything.
The 2007 client is a cool client, but it has horrendous security issues. That's why it's being removed - it's ridiculous how many vulnerabilities it has. It (or any older client we happen to find) are never coming back - if 2007 is already this bad, imagine what 2006 would have been like. I hope you understand that 2007 is a horribly unsafe client, and you are literally risking your entire computer by playing it online.
You can play 2012 - and yes, we're aware of the Windows 7 bug on 2012. Don't quote me on this because I'm not a client developer, but I assume that bug will be fixed soon. We're not sure if we will ever add another "classic" client again (pre-2010), but if we do, I assume that it'll be 2009. Again, I'm not a client developer, so don't take my word for that.
Hopefully you understand our reason for removing 2007 now - it's fun, but it's a terrible, laggy, ridiculously vulnerable client. That's why it's being removed from Finobe and never coming back.
tl;dr: you are at risk of OBAMA BALL up your computer if you play 2007