What the heck is this?
This is a small guide intended for IP poor players who cannot afford to have multiple champ rotations to accomodate the team. Say, your single favorite champ is AD(Attack Damage) but your team picks AD heavy champs themselves. Wouldn't it make sense to switch to an AP champ to avoid Frozen Heart and armor hoarding by the opposing team?
AP(Ability Power) Janna's effectiveness is maximized if the opposing team has almost zero MR(Magical Resistance). If you still don't like Janna you can pick any other AP champ you want instead.
Since this guide is intended for newer players I will explain how to determine wheather your team is AP or AD heavy. Use in game champ info and online champ databases to determine this. Take a look at each ability to see if it does AP or AD damage.
First, pick out the champs which you know to be 100% AP or AD. For example, Tryndamere and Pantheon are purely AD. Do the same for AP champs.
Then, take into account champs that are a mix of the two. Be careful here as many champs that seem to be purely AD or AP can actually be more of the opposite( Rammus, Shaco, Twisted Fate and Alistar are only a few examples).
Lastly, juxtapose all 4 champs your team picked to determine if the team is AD or AP heavy.
If it is AP heavy, then pick pantheon. Otherwise, Janna. If it is a balanced team then it doesn't really matter much which champ you pick but it always makes sense to balance it even more with such champion as Blitzcrank.