IMHO the reason we see so much "hour building" interest is the artificial way the ATPL is structured - particularly in Europe where it is a CPL/IR followed by a load of hour building to reach 1500 TT including (AIUI) 100 at night.
If we had the concept of "demonstrated competence" then hour building would not take place. Hour building usually involves renting the cheapest possible spamcan and flying back and forth - or PPL instructing - and thus has practically zero relevance to the ATPL which is generally about carrying paying passengers in a highly automated multi crew cockpit.
I don't really know who you have met or spoken to, but I really don't see hour building practiced in this way. For a start, the term "hour building" most usually refers to the hours post PPL to qualify for CPL training. Post CPL/IR the time to 1500 hours is for the ATPL license. This time isn't hour building, typically it is actually commercial time which must include multi-crew and has nothing to do with the MPL (referring to other post). If anything, the cited concern with the MPL is that it reduces actual flying time considerably and just, so the argument goes, the ability to excersise sufficient command judgement.
In terms of "demonstrated competence", that is what the CPL and IR skill tests are for, not to mention the sim check, followed by the type rating, the license skill test, then the line training, the recurrent LOFT, OPC/LPC and line checks. So one doesn't get to hold an ATPL without completing at least 23 exams and 12 official skills tests and checks!
It irks me to see threads on "building hours". How about building experience, the hours will fill themselves in while you're doing that!
Just to provide a counter point, hour building (in the most recognised sense) with the major FTOs is a structured exercise aimed to build the trainees experience. The self-managed modular chaps I have met generally look to try something different each flight. One chap really made the most of his IMC rating and, I would say had much more experience in his first 150 hours than many PPLs achieve in 500.