First off, I posted a kind of summary of this theory in the thread about the episode, but I decided to go into more detail. This is about the first half of the episode, specifically the spell.

I believe that Starswirl's spell was meant more as a lesson than an actual spell for practical use. I believe that the spell is meant to show that even through magic, you cannot truly change the destiny of somebody. This is supported by the fact that having the ponies do what they were meant to changes their cutie mark back. It could be a lesson about being yourself.

Let's take a look at the spell itself...

"From one to another, another to one. A mark of one's destiny singled out alone, fulfilled"

One thing that could be noted is that it says "A mark of one's destiny singled out alone", perhaps implying that one's destiny is unique, and that even though you can change what their perceived destiny is, you can not change what they are meant to do.

This could be about the real life situation some people face wherein they try to be something they are not in order to receive praise from your peers. It might mean that you can not change who you are, so you should be who you are best at being, yourself.

And that since they need each other, it is to show that your actions and duties affect many more than just yourself.