Definitely not saying it was useless, just that the utility was limited because of the usual limitations of MDA; that is, it's definitely not the same as what people would think of as a dual-head setup today.
That said, I literally was born after Windows 3.0. So... I never personally dealt with such a setup. Maybe some day, as a matter of exploring older computers, I will get a chance to actually run such a setup and see what it was like.
It was definitely a thing to run multiple screens in a "one-per-task" kind of setup - the real difference is that hardly anything back then networked or multitasked, and so you had to design the workflow you wanted to use specifically around the idea of using all your displays. In the context of any specific application(coding, document editing, CAD etc.) the possibility of a custom tailored solution existed - it just wasn't the highest priority most of the time. You could get a lot of multitasking mileage out of more antiquated technologies like printed documents and pens in tandem with your computer. "Code review" meant literally printing it out and going over it with highlighters like an exam grader. I'm sure it still does in some shops.
That said, I literally was born after Windows 3.0. So... I never personally dealt with such a setup. Maybe some day, as a matter of exploring older computers, I will get a chance to actually run such a setup and see what it was like.