I have some experience with this topic, having helped set up the 401K plan at a former employer.
I recall there two standards in the industry: (a) the suitability standard for brokers; and (b) the "best interest of the client" for advisors.
The unannounced third standard is: managing investments yourself because you don't trust anyone else to do it, fiduciary or not. I like this third option the most.
This arises with 401K plans because the insurance company representatives who visit employer sites to sell 401K plans are paid by the insurance companies, not the investors, and therefore don't have to act in the best interests of the investors. An investor sued one such representative, and his defense was just that: he didn't have to observe the fiduciary standard, and therefore could not be liable for violating it.
While the suitability standard is a joke, I feel fairly strongly that the fiduciary standard is as well. One danger of the dispute over the fiduciary standard for brokers is that some people will inappropriately become complacent about the behavior of advisors simply because the fiduciary standard applies.
A friend of mine has a low 7-figure nest egg with a large financial firm which is managed under the fiduciary standard. The sum of the advisory fee and the ERs comes to 2% per year. Between the age of 25 and 65, a fee at this level reduces the final nest egg value by about 40% from what it would have been, had it been invested in low-fee index funds. The final payout will be reduced still further (by half in fact), if the adviser skims 2% of assets from a 4% withdrawal rate.
If an advisor, while in compliance with the fiduciary standard, can leave a client with about 1/3 of the retirement income that would have resulted from investing in index funds with a reasonable level of competence, and not paying fees, then the fiduciary standard is worth little if anything as I see it.