I'd have to argue that "religiosity" would have to be measured by something other than nominal affiliation or polls alone to be an accurate descriptor.
For example, if a person nominally identified as "vegetarian", but eats meat 364 days out of the year, then in practice they would arguably be less "vegetarian" than a person who nominally identifies as a "carnivore" but eats meat only 1 day out of the year.
Since environmental degradation, for example, would be antithetical to the major world religions - one could potentially make an argument an American "religious" who ignores environmental sustainability is less "religious" than a European "atheist".
Using frequency of "prayer" to measure religiosity also strikes me as problematic, since this would equate "mythic-level" prayer in which it is done perfunctory by rote from higher forms of prayer or spiritual practice, such as mystic or monastic-level prayer and meditative disciplines.
Mankind is posed midway between the Gods and the Beasts - Plotinus
Well, I would argue they would have to exhibit some level of day-to-day practice of religious concepts, such as non-violence, healthy lifestyle choices, purposeful living, environmental responsibility, putting higher pursuits above purely material concerns and pleasures, et cetera.
If it is reduced to nothing more than what they nominally their views are, then I fail to see how it means anything more than what sports team a person says they're a "fan" of.
Mankind is posed midway between the Gods and the Beasts - Plotinus
I agree, but my argument is that it would have to be actualized in some way or another, not necessarily "materially" or externally, but internally as well.
However if a person "identifies" as religious but in practice spends the majority of their time on energy solely on material things, then I'd have to argue that their level of religiosity isn't the same as that of, say, a practicing monk - essentially my argument is that not all levels of religious are equal, just like not all levels of "fitness".
Mankind is posed midway between the Gods and the Beasts - Plotinus