Well what is the argument here? That it doesn't "make sense" for spellcasters to multiclass based on effectiveness, or personal bias? Effectiveness is highly variable, but I've seen Sorcerer/Warlock builds that game the system and don't seem terribly upset by the loss of spell slots.
Personal bias? Everyone has something that triggers them. 2017 and I still know people who are put out that Dwarves can be Wizards or Elves can be Paladins. The most common complaint comes down to a lack of justification based on the story.
If a DM wants to say that characters can't have multiple 'power sources' because it's not done in his setting, hey that's fine. But to say there's no logic in a character already drawing from one source to seek out another is overly simplistic. There are lots of reasons why a character might do such a thing. It's entirely possible, for example, to have a Fiendish pact with a powerful Archdevil, like Asmodeus, who actually has Clerics in some settings.
So a Fiendlock is offered new powers from his patron, and gets Cleric casting? Sure, why couldn't that be a logical progression for a cultist?
Heck, at that point, he could cast cleric spells using his Warlock slots, and while he won't have as high level spells to call upon, he can cast cure wounds all day long! So now we have a situation where multiclassing as a caster can both be effective and justifiable.