I thing it would be great for demonstration/comparison purposes and providing a quick fix for titles that have issues with playing in-game videos/cinematics such as Goldeneye: Rogue Agent. It's a great QoL feature in PCSX2.
In PCSX2, the software renderer is actually intended to be useful for regular people, so it makes sense to make it easily accessible. In Dolphin, though, the hardware-accelerated backends are all supposed to actually work in all situations, and any effort making the software renderer fast enough to be playable in the rare cases where the alternatives have bugs would be better spent fixing the bugs. There've even been cases in the past where bugs existed for a long time in the software renderer after they were fixed in the other backends.