Hi aprendiz,
That's not a good idea. Since you will need to modify ALL of the links and options in the admin panel to have this new param.
For example, all the links in the admin menu lack of it, also, the login form,.. so you will be able to get the login page, but not to be able to perform login.
I think the admin is secure enough, of course, if some security flaw or improvement appear will fix/add it, but we're using the standard method for it.
Thanks