I still remember my first discussions about the trend for painting in a darker or contrasting root.It was perhaps a couple of years ago, and I was at the salon, pretty gobsmacked that people would be starting to request freshly coloured hair, with a faux-root added in. I couldn't imagine people wanting to pay extra for something we fought so fiercely against.
I laugh at how wrong I was, in the same way I used to scoff that mobile phones would take off (why would we carry one when there is a payphone on every corner?!).
I have come to love a dragged down root for a couple of reasons. The first is that it can make your rainbow coloured hues look more 'expensive' or classy. If you've been to a salon to have your hair down, you don't want it to look like a bad home-job. Creating a quality root it a skill, and most people couldn't replicate it at home. Swish that salon do with pride!
Another reason the exaggerated root is great is that it can prolong the time you can leave between appointments. You will still see your natural regrowth, but it won't look as obvious, meaning you can save some cash and wait a couple of extra weeks between a fresh tint. It can also be used to bridge the gap between growing out an old colour, by creating a more obvious root, you can skip the awkward growing out the root stage.
Lastly, it's just an added way to add some fun and dimension to your hair. The more colours can be the merrier.