I'm not sure I follow. How does identifying with multiple religion undermine intellectual rigor and discipline? I'm not seeing the connection there. If anything, it seems to me that identifying with multiple religions would require more rigor and discipline, not less of it because one is working with multiple traditions and there is more stuff to keep track of. Though when it comes down to it, how much work one puts into one's religion is a matter of preference, and can fall across a broad spectrum no matter what one is identifying with.
By my own anecdotal observations, I agree with you, though I wonder if a formal survey would uncover with respect to this. I always like to go with the data when I can, though interpreting that data can be a bit of a challenge too! 
To be clear, with respect to how PEW does their surveys about religion, the "spiritual but not religious" crowd is within that "none" category, because they identify as "no religion." "None" in terms of religion does not mean atheist.
I'm trying to think of how to explain it, but if you look within one religion-say christianity- you have literalist interpretations of the bible but can also have liberal interpretations as well. The difference is "who" has the authority to decide what the "correct" interpretation is: "god" (or the church and the preisthood) or the individual christian. Ultimately, either one or the other will take precedence and a believer could not cliam both as equal given that the practical implications is it makes conflict of interpretation unresolvable. They overlap, but are mutually exclusive and incompatable. (I think this came up in the protestant reformation and the same battles over who can interpret the Quran are being waged between liberal and orthodox Muslims).
The history of individual religions is full of these conflicts over interpretation over what practice or ideas are accepted and based on whose authority. A religion is a sort of collective memory and body of knowledge which passes down those traditions, practices and interpretations. Knowing one religion passes down this history as lessons for the present day. I think even most fundamentalists agree on the need for
some interpretation such as to respond to wholly new situations such as technological changed e.g. Is using internet pornography a sin? Can you wage a "just war" with nuclear weapons? Is abortion immoral? All of the above would not have been directly covered by the bible or the quran as they are recent technological changes, so believers have to
infer what they consider to be the "correct" or acceptable interpretation and someone has to have the power to decide if that is the "true" reflection of existing doctorine.
Christianity's struggle to reconcile the book of genesis with scientific discoveries such as the big bang and evolution is an example of how certian beliefs can be mutually irreconcliable. Should we treat genisis as a myth or a fairy story and give priority to the authority of new scientific discoveries, or is genisis god's unalterable word and should we fight the corruption of christianity by atheistic-materialist science?
In the end, its the fact that religions have their own politics. Because of the authorities behind it, these can tend to be closed and self-contained traditions. Adhering to multiple religions may mean accepting many conflicting authorities or claiming to be one yourself to interpret the religion. If someone tries to adhere to both Islam and Christianity, eventually they're going to have to "pick sides" on the debate over Christ's divinity or the reliabilty of the Quran.
Adhering to multiple religions ultimately places the authority to interpret those religions in the hands of the individual believer. They may not know enough about the religions they follow to actually make a coherent choice. I guess I'm saying that they cannot follow "two masters" as my understanding of what religion is more collectivist/conservative than individualist/liberal.
