I don't understand why schools force their students to do general studies/critical thinking when universities tend to exclude them from offers?!
In response to your question, some people enjoy more than four subjects and thrive on the challenge whilst others may do it for a more impressive UCAS form. There are so many different reasons, it's all about personal preference at the end of the day since what suits one person might not suit another. To be honest, if someone has the willpower to do five and achieve straight As, they may as well go for it. However, no one at my school did five AS levels in one year and offers ranged from Oxbridge to Imperial, etc, which just goes to show that taking the standard four doesn't harm anyone.