OK bhavv stop spreading misinformation please. You clearly have no idea on balance whatsoever. Just enjoy civ5 as the civ game that has tried to make it the most balanced so far.
Floodplains with loads of wheat tiles are already attractive enough to settle with or without desert folklore, they dont need boosting even further.
Floodplains do NOT get bonus from Petra, only desert folklore, so your screenshot on the city with lots of wheat + floodplain is NOT a good candidate for Petra.
Furthermore in civ5 floodplains are merely grassland yield wise, so I dont think you fully comprehended the statement you made saying: "Floodplains with loads of wheat tiles are already attractive enough" since that is the same as grassland with wheat, or grassland with cows/sheep (which are in fact better imo).
There is a difference though:
1) Cities with floodplains compared to grassland have the downside of having nearby desert tiles which are practically useless
2) Getting bonuses for desert such as folklore will be limited to that one city, if not max 2-3 under best circumstances. However bonuses to other tiles such as quarries are likely to apply to a much broader number of cities
3) Consider the production investment of bonuses such as Petra, consider the opportunity cost of it, if you'd built a stonehenge (which is much cheaper too) you would be getting a +5faith (equivalent to 5 desert tiles being worked), or perhaps a great library.