DPS Janna may be viable, but it is definitely not the best way to play her. Here's why:
1. None of her abilities scale with AD. That means that casting Eye of Storm on Miss Fortune, Ashe, Corki, Jax, Gangplank, etc is significantly more effective than casting it on yourself.
2. Even if you compensate with HP Quints and Defensive masteries, you are still squishy, very squishy. Janna has several built in mechanisms for dealing with her squishiness, namely her speed and her abilities that knock enemies up and away from her. None of those work if you're trying to stay close to autoattack.
3. Janna is strong well into the late game with very little gold. With nothing but Doran's ring, Nashor's Tooth, and boots of mobility, she is still the strongest support in the game, and will net your team many kills. On the other hand, whoever she lanes with likely does not share this trait. Minion kills are better given to them. This is also why Janna should never take mid or the solo lane.
I could go on, but those are the main points.
Now for some other things I noticed...
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