As to the point about mindsets and fluff: I don't choose to negate them, I combine them. With my recent PC, a level or two of cleric does not seem so alien for a celestial pact warlock.
Where I bolded your comment: THIS is what I would call the key determinant for supporting or objecting to a multiclass. In your case, I agree entirely. Worshiping as a cleric could naturally lead to a closer and more personal pact with a servant of said deity. Heck, you could even argue that since the clerlock has an active relationship with a being who speaks directly to his deity, he actually grows more in his devotion to his deity as a result -- the polar opposite of having a jealous god who objects to the warlock pact. It's all in how you frame it. And in this case, I'd agree that is a reasonable path to take (even if I'd try to avoid it).
Without that natural/organic pattern, though, too often MC simply looks like an attempt to game the system. To me, at least.