Anyway, pennyless Janna is very powerful, yes. Not arguing with that. The 4 others champs to fight thing is a different matter. Are you seriously suggesting you shouldnt kill the carry? If she deals all the damage, then she gets all the kills, and then she gets all the money. By killing just the one person, you could be taking out about 45% of the teams power.
I have seen games where a carry could have easily killed everyone with her damage, but fucks up ONCE and gets killed. And then the team cant do shit. The other team just barons/pushes an inhib, and wins. A coordinated team could keep the carry alive, but you dont exactly get that in non 5 man teams.
All Of the 4 other champs basically soloed. You have jungle, top, mid, and bot with Jana. That means every carry, and even the tank, soloed on your team. That means more CS, that means more gold.
Of course they should try to throw everything at your squishiest team mate. But to do that they have to (in theory) get past a fed tank, survive a fed AP carry, and stastain a support's CC. If they do get the AD carry, you still have 4 champions alive, fed, and mostly full on life.
For instance, I played almost this exact scenario. Ashe and I (jana) bot. Her arrow + my CC resulted in many early kills. Ashe got all the CS. At the end game we were crushing them. Their entire team would dive ashe, kill her, and wipe. We asked them why and they said "Because Ashe is the only one Jana can't keep alive." That was fine by Ashe! 1 carry for their whole team is a great trade.
Anyways, like I said before, it only works with the right balance. Your tank has to be one of the tanks that can solo a lane. Your AP carry has to be able to solo a lane. Your Jungle has to be able to gank and you have to have two AD DPS already. If your team does not have that setup, it isn't as good of idea to pick a support.