No, no, we understand how Servants can be summoned in different classes. The question was regarding the justification for Caster class in particular as a supposed 'best class' more than anything else.

And 'Noble Phantasm' and 'Skill' refer to two different things. There are active and passive Noble Phantasms, but a 'Noble Phantasm skill' is just a confusing way of referring to... Anything, really.