Intelligent MPU-401 does not make games sound better or worse, it allows games work with midi devices like the MT-32, otherwise they do not work with them at all.
That is not quite correct.
An original MPU-401 has its own CPU and simple MIDI sequencer functionality onboard.
Intelligent mode allows software to offload some of the MIDI processing and timing from the main CPU to the MPU-401, allowing for lower CPU load and higher accuracy.
The communication with external MIDI devices such as an MT-32 is no different, regardless of the mode used. So anything you can do with an MT-32 (or any other device) in intelligent mode, you can also do in dumb mode.
Which is also why a program like SoftMPU can work: the actual MIDI interface still operates in 'dumb' mode, because that's the only thing it understands. The 'Intelligent' part is just emulated between CPU and MIDI interface.
In practice there is a close link between intelligent mode and the MT-32, since the MT-32 was the first MIDI module, and the MPU-401 was the first MIDI interface for PCs. They were often used together, and many early games would use the intelligent mode.
When clones arrived with their own MIDI interfaces, they only supported 'dumb' mode, to reduce cost (so the onboard CPU and sequencer functionality were dropped). As a result, many later games no longer used intelligent mode (also because faster CPUs made it less important).