[libvirt] [question] Why doesn't virsh command support an optional parameter

[Zhang Xiaohe]

hi guys,

It's hard for me to understand why doesn't virsh command support an
optional parameter because I think omitting a parameter and offering
a default value sometimes is quite convenient.

For example:
$ virsh shutdown guest --mode acpi
The option --mode only accept string "acpi", "agent", "initctl" and
"signal", assuming that "acpi" is the most frequently used parameter
when --mode is specified, why not just using
$ virsh shutdown guest --mode acpi
instead.

Maybe this example is not very precise, but what I mean is if there is
an option which only accepts several candidate values and one of the 
value is always used when it is specified, then we can make it the
default parameter of this option. This would be convenient and save
some typing.
Of course, this can also result in ambiguity to people who is not
familiar with the command. But with the help, it won't trap one very
long, will it?

So, why doesn't virsh support this and what do you think of adding
this feature?

Thanks
Zhang Xiaohe