No, you shouldn't.
There are real liability and security issues associated with the installation of unsigned, untrusted applications, that effect you and how you use "your" mobile.
For example, there is a legal requirement to be able to dial emergency services... even if you phone is on a network it's not authorized on. Even if your phone is locked. The requirement is that anyone can pick up your phone and use it to call emergency services.
Now picture, for example a virus that propogates between phones (on unassigned apps., naturally) and all it does is prevent a user from being able to dial 999/911 etc. Such a virus can result in people dying that would otherwise be helped.
Even forgetting the dramatic, how would you feel about having to install and run a virus scanner, and check constantly for new virus updates? How would you feel about viruses that run up costs on your phone, unknown to you?
The security model in Symbian OS allows for manufacturers, operators, and enterprises to all have their own trust (application and device management) domains. It's far more flexible and secure than MS Smartphone, or the Blackberry platform. When you consider that the smartphone market will grow to $1B+ devices (and Symbian is still forecast to be more than 60% of that market) by 2012.... a few of those users are not IT savvy, and if they have security issues, like viruses on their phones.... they'll end up calling their operators for help.
And those calls to operators for support result in higher bills for you and me.
So, security is an issue that effects all of us...our safety and our pocket books. Be thankful that Symbian is the most popular and mature of the mobile OS's from a security perspective.
