What's the best way to degrade gracefully when using a Plugin Variable?
I think the best way to do this would be in your PluginHandleVariable file's method, just have it accept one, optional parameter and expect it to be an array.

public function variableMethod($options = null)
{
    if (!$options || !is_array($options))
    {
        return "No options set";
    }

    // Now you can check for individual options on the array
    if (isset($options['option1'])
    {
        ....
    }

}


Then both of these would work:

if craft.pluginname.variable()
if craft.pluginname.variable(options)