I think different hair styles suit different ponies, and sometimes it's fun to just play with the hair or stylize heavily... thinking something like Art Nouveau here, or just hair that's so long and thick and luscious it would be impossible in real life. Whatever feels best at the time!
Of course it also depends a bit on the background/scenery, too - perfectly neat out of factory/salon hair usually looks more convincing on an abstract background (or none at all), while naturally flowing or messier hair fits outdoor pictures better. Real horses often have tufts of longer and shorter hair, their manes are not perfectly even, so sometimes that layered cut style may look more realistic. Then again, there are real horses with manes as long and perfect as those on Barbie horses, so... I think everything goes!