That's the main problem - if you play your cards right. If you play your cards wrong, you end up with a terrible day.
If you don't know the park, or just go to the wrong ride at the wrong time, you can lose a third of your day focusing on one ride. It's a dreadful experience.
Of course, if the park is truly quiet and most rides have next-to-no queues, 6 hours is plenty. But if it's vaguely busy, it's a nightmare.
On the positive, it's good to see the parks extending opening hours. But ultimately yes, Towers' 10-4 days have long been silly and really, 10-5 should be the minimum for the park. I've also never liked the logic of announcing planned extended opening at the last minute - surely you want people to know your opening hours in advance?! Fine if they decide to extend on the day because it's busier or reliability was poor, but if weeks in advance the park know "We're going to close at 5 this week", why wait until the weekend before?