To tell a person that THEY should choose a pantheon to work with is completely arrogant. To me that's like telling a god "hey, I'm going to worship you and you get no say whatsoever in this". And yes, some people do this and it works fantastically in the end and they develop an incredible relationship with their deity. And for those that see all deities being part of the lord and lady/spirit, it's weird to me that you should think you have to choose one face of the Goddess over another anyway.
My point is that by making people who are just starting out on this path think that they "have to" choose a pantheon causes many of them to try to force a relationship with some deities, and ignore others because they aren't part of their chosen pantheon. I honestly think the attitude we should be "teaching" is that they should be open to deity in all it's forms. If a whole pantheon calls you, cool beans. If Aphrodite, Kali, Aset, and Krishna all call you, that's pretty awesome too. I think this should especially be expressed to people seeking to follow reconstructionist paths because I've known many who choose their pantheon, decide the best way to worship them is in their historically accurate form, and then try to force a relationship with a deity who they have a terrible connection with just because "historically" they would have been a main deity and honored every this day or that. Many of us came to Paganism because we were tired of mainstream religions telling us who, how, when, and where to worship. While the "you have to pick a pantheon" attitude isn't quite the same thing, it's not to far off to me.