Slice defaults with multiple values do indeed only work with flags at the moment. I think this is because flags support sep:"," while positional arguments don't because they're pre-separated from the shell.

I'm not sure what the right solution here is really, syntactically. It wouldn't be logical to allow sep:",", and splitting on space would preclude having an argument with a space, such as in this example:

Strs []string `arg:"" default:"foo bar"`

One option might be be to support multiple invocations of default:"X" eg. default:"1" default:"2" default:"3", though it is a bit verbose.