Some creature mods are more useful than others, but no mod is ever useless, I'm convinced. It's just about the number of use cases and their popularity. This goes for both shields and foci. (Also Jonast please keep in mind usefulness and price are not related in gw)
But yea, why exactly these creatures were created as mod options is a mystery. I get the charr/dwarves/undead/plants/demons, as they were the main enemies during some part of a campaign's storyline. But giants, come on... There are so few and none of them are important to the game in any way.
About skeletons, at least those were worked out a little bit more, like extra armor against piercing dmg, it's logical (hence also the necro bonelace insignia). I suspect that the skeleton mod idea appeared earlier in the development process, and anet didn't bother to remove it once undead was implemented.
The rarest shield combo in my opinion that still drops would be +10 vs creature +1 attribute 20%.
That's not a matter of opinion. And even without the "opinion" part, that statement is false. It's not even second rarest.