Don't get me wrong - overall, I like the class, and I see where you're coming from on this topic. However, this part of the design does seem contrary to the design philosophy of other FC classes, which generally spread the attribute dependency from class abilities (so that the Priest has a slight Cha-dependent ability in path casting, and a Wis dependent 10/20, for instance). This is the only attribute-dependent class ability the Warlock has (the benefits from other stats being only of the generic variety) and it's dependent on only one attribute; hence the minor issue.
Yet on the flip side of your argument on it being a FC standard class design rule, I have to argue that I don't see this is as a part of the core design philosophy at all when I look at the core classes, as so many classes don't fit the mold you describe.
[Assassin:[/b] Lists 3 favored attributes (CHA, STR, WIS) and more importantly has
no full class abilities that trigger off
any specific attribute beyond the core attribute functions. The only aspect of the class that comes close is the selectable ability "Follow My Lead" that relies on its Primary favored stat (CHA), and on top of that there is a good chance it will be 1 of the 3 selectable abilities that doesn't get chosen.
Burglar: Only lists 2 favored attributes (DEX, WIS) [vs. Warlock's 3] and more importantly has no full class abilities that trigger off
any specific attribute beyond the core functions such as DEX for REF and Defense. The only aspect of the class that does, is a selectable ability (Bloody Mess / WIS) and even then there is a good chance it will be 1 of the 3 selectable abilities that doesn't get chosen.
Captain: 3 favored (CHA, STR, WIS) but has
no class abilities at all that trigger off an Attribute score.
Courier: 3 favored (CHA, WIS, DEX) but has
no full class abilities that trigger off
any specific attribute beyond the core attribute functions. The only aspect of the class that comes close is the selectable ability "Slanderous" that relies on INT, and on top of that there is a good chance it will be 1 of the 3 selectable abilities that doesn't get chosen.
Explorer: 3 Favored attributes (INT, DEX, CON) but has
no class abilities at all that trigger off an Attribute score.
and the real "design breaker"...
Keeper: 1 Favored attribute (INT) and No class abilities at all that trigger off an Attribute score other than INT (Instant Solution - though even that is a stretch).etc.
So, maybe I'm failing to see the true point of your argument, but to me I see plenty of examples of classes (almost all of them) that would support letting the warlock's Invoking rely on its primary favored attribute instead of trying to force a mechanical reliance upon more attributes beyond what the class needs other attributes for, for their core functionality (BAB, saves, Defense, initiative, skills, etc.) such as Soldiers really needing nothing more than STR and CON.
JMHO.
Morgenstern: Its not that hard to move the saves bonus from Cha to something else is you want to split up the attribute focus Smiley. Cha to cast, Wis for saves...
But is it a design necessity?
Edit:In the end, if its really THAT big a problem, I can always just drop the Invoking concept as a skill and go back to making Invocations act like Natural/Divine spells and then by Core Rules, they only rely on 1 stat anyway - CHA.
Personally I think it works fine to have 1 niche class that is specifically designed as a hybrid between Priest (
minor reliance on 1 attribute) and Mage (
major reliance on 3 attributes) to have a balance of attribute reliance that is a
major reliance on 1 attribute.