MLIR dialects have to be lowered into the basic LLVM one eventually, don't they? Does MLIR add anything over a custom IR for host languages that aren't deficient at manipulating data structures?

'MLIR dialects' is just a term for teaching MLIR how to manipulate and understand your own custom IR.

MLIR is just very good at producing good vectorized code in the presence of stuff like nested loops compared to LLVM or even some of the most carefully crafted custom compilers. It's not about whether your custom compiler is 'deficient' at handling data structures, MLIR is just genuinely very good at some of this stuff compared to basically anyone else.

For most projects it's just more trouble than it's worth though, because maintaining and using an MLIR dialect definition is hard.

But AFAIK those aren't features of MLIR, but of lowering to existing MLIR dialects and running their passes. My genuine question is whether these passes provide any benefit before lowering, because otherwise a custom dialect doesn't add anything over lowering from a custom IR for anyone not using C++; and the only example I've seen is forced inlining.