That's the obvious solution. But, sometimes if you're working a design, and just want a section that is specifically a color, or varied without a part - using a section of the overdye gives a great (perfect) match. Even if you go to the base color - which is sometimes doable - it will look a bit different. So, that's a reason. And besides, the overdye may just be what's readily "right there" and handy.
Also, there are indeed some designer/teachers who have made it somewhat of a custom to manipulate overdyed threads. In general I agree, that if you don't want to use the overdye as the lovely color changing, graduation effect, then why not just use some solids. But, as the saying goes, different strokes for different folks. OTOH, it is a pretty neat effect when you can do mirror imaging with them, etc - just playing with what you have to see what turns up can bring some interesting art to light - as you yourself do with your fiber art.
Ellice