This was touched on, but I don't think the person putting a system together with a $300-$600 motherboard is also going to be putting in a GTX 1050 and some low end CPU. If they did, then yes, of course they would be better served buying a new CPU and GPU, and they shouldn't have bought an expensive board in the first place.
But again this is also the premise of the gamer / casual overclocker. There are definitely different demographics here and how the motherboard plays into their needs is going to vary as a result.
Gamer - Just wants to turn it on and play a game. Likely comes from consoles. Doesn't know anything about hardware, has never messed with a BIOS, and has no desire to. Absolutely this gamer should not bother overspending on a motherboard.
Gamer / casual overclocker - This is the gamer who also knows enough to tweak the system for decent enough performance gains in gaming and probably will want a board to support what they want to do, but being casual, still probably doesn't need a super expensive board. May want RGB, etc.
Gamer / enthusiast - This is the type who knows what they are doing with PC hardware, builds a system that makes sense in its config - i.e. a high end system with high end CPU, GPU, etc., mid-range system with good mid-range config, etc. They will want the convenience features, the easy diagnostic features, they want a board that will support memory overclocking and tweaking, they can play with voltages, boosting, etc. They probably want to bench their system in addition to gaming, but they aren't looking to build a test bench. It's still a daily use system, but also might be a system that has multi-use purposes for professional uses in addition to gaming so they might want the better I/O, networking, etc. Here you probably do want a better than average board.
Enthusiast - Building a system for benching only and wants all the high end features to support that. Wants to be able to easily swap parts, is probably putting it on a bench. Wants support for things like LN2 cooling, etc. They want the super high end boards with the features to support this, 2 DIMM, etc. I mean this would be really what the more expensive motherboards are for.
So I guess that goes back to which demographic is the premise of this thread referring to when asking the question. In my opinion there's tons of nuance here. If all you want to do is enable PBO and XMP/EXPO, then sure don't overspend on a board. If you want to actually get into water-cooling and tweaking your CPU, memory, etc. then perhaps a better board is warranted. Want to do more enthusiast grade benching and other things like that? Then yeah go all out. Even if not doing benching but want certain I/O and networking features that the motherboard manufacturers have segmented arbitrarily into higher priced segments? Then yeah gotta pony up.
Sadly I think the motherboard makers are all in cahoots for how they segment even convenience features like a post code display and push button clear CMOS into $300+ motherboards when 10 years ago I could easily get that at the $150 tier. Just another way we are all getting screwed in the PC hardware space in addition to rising GPU pricing, and other shenanigans.