Two "benefits" can be derived by combining magic pins:
1. The magic lasts longer.
2. The magic takes less time to regenerate.
It's one or the other depending on the magic.
For instance: regular 1-star Fireworks lasts about 10 seconds. 5-star Fireworks lasts about 30-40 seconds.
It's up to you whether you want do combine, but it's an irreversable decision. Once you combine, you cannot break them apart. One thing to keep in mind, if you have multiple magic pins, then you have some good trading material.
If you're really into magic, then combining may be a consideration for you. If magic isn't really a big deal to you, then you might want to keep them seperate in case you want to trade.
"Yesterday, a man walked up to me and said, 'Isn't it a shame that Walt Disney couldn't be here to see this?' and I said, "He did see this, that's why it's here."
-Art Linkletter July 17, 2005-
I don't combine really rare magics, as trading them would probably be better, so I am left with one.
I DO combine Fireworks, since I can always get more
one time ( before dresses were re-released for halloween ) I got a blue dress top and shoes ( already had skirt ) for 5 Star Fireworks, which worked since I <3 the game and get many magics anyways!
So its up to you for what magics you combine, and what you trade for!