It is not necessary to use phonemes in the model, however, it greatly restricts model's ability to learn, in particular to learn many special cases like rare fo

It is not necessary to use phonemes in the model, however, it greatly restricts model's ability to learn, in particular to learn many special cases like rare foreign words. You need much more data to train the model and basically you lose ability to quickly teach the system a new word by specifying word pronunciation. It might sound great for advertising of computing power factories like Baidu is doing, but speech recognition experts are not that enthusiastic about such approach. It is pretty disappointing that modern machine learning "experts" advertise their improved results with questionable comparisons and do not care about machine learning theory, in particular, never consider model ability to generalize, model robustness to noises and related things which must be primary subjects for research.