I disagree strongly, it all depends on the champion, when you got Support spell heavy champions such as Soraka and Sona a AP support build will be more beneficial to both you and your team because you get more power behind your support spells, unlike an AD- or AP-carry where what you should focus on building is very clear a support can be played in a variety of ways depending on champion and play style, Sona is a support champion that can be built AP support because a strong heal helps the team allot and as long as you make sure not to pick up kills with your aggressive spell.

personally I prefer to play Sona more aura based, but I still like to pick up a good AP item for some extra power, I will try out this guide the next time I play and will give you a feedback after I have done so, but as it stands now I think you should take some time to explain things a bit better, go more in depth basically and explain yourself a bit better.

As I said in the guide this is my opinion only. My opinion is that Sona can do her jobs better (heal and harass) when she has items to strengthen her abilities :)

The roll of support is to stay back heal/ buff team mates while harrassing the enemy team. Sona as a support is not supposed to buy any ap at all. Supports in genral should only build aoe iteams that help thier team mates.