It is a wise practice, IMHO, because anyone who spends a significant portion of their life road cycling is subject to fall victim to someone else's accident. I just don't fancy the expense or the execution.
There isn't much information that your RoadID could provide that would affect how the EMTs/paramedics will treat you, unless you have an especially rare medical condition or allergy. Blood type in particular is purely eyewash because the protocol is to type and crossmatch before administering it. EMTs/Paramedics do not carry blood (it is too scarce and too perishable), much less administer it, and no competent medical professional will trust what is printed on your ID, regardless. If your condition is so critical that they cannot wait for for lab results, they administer O-neg rather than trust even what you might say, provided you are conscious, or even what is tattooed on your chest (have you never seen a misspelt tattoo?).
I have a couple of rare drug allergies, and an unusual medical condition that could affect diagnosis, so I wear a Medic Alert necklace. But that information only would be beneficial if I were uncommunicative for a lengthy period and no Next-of-Kin was available.
STODRR's example of surgically-implanted steel is an excellent point (all mine are titanium, which is non-magnetic). Depending on your injuries, that is something the treating physician might need to know immediately (but, again, not the EMTs/Paramedics).
So for the typical "No Known Allergies" cyclist lacking a steel plate in their head, the primary benefit of the RoadID would be in the providing of NOK information, or long-term treatment information (which probably wouldn't be necessary if a NOK were available), or the existence of your "living will" or advanced medical care directive. If you are unconscious and no NOK or AMCD is available, doctors always will assume that you would consent to being treated as they see fit.
When I cycle, I leave the smartphone at home and carry a small clamshell mobile phone with an ICE number programmed into it. But it cannot be relied upon to remain serviceable after a severe crash so I also carry an emergency ID of my own manufacture, kept along with the mobile inside of a snack-sized (3x6"/8x16cm) ziplock plastic bag in the back pocket of my jersey.
If you think your bloodied and shredded clothes are summarily discarded by hospital staff, you are mistaken. One of the EMTs' duties after the drop-off is made is to transfer any personal effects to hospital staff, who will search them, specifically looking for identity documents. So the fact I don't wear my RoadID-I-Y around my wrist or neck is inconsequential.
But I cannot justify the expense (or, to a lesser degree, the lack of timeliness) of a RoadID because I can create an equivalent document myself with a computer, a printer, and a trip to the nearest business offering laminating service. Your address can change, the identity of your NOK can change, your NOK can change telephone numbers, or you might change insurors or insurance policy numbers, any of which would mean your RoadID is "out of date."
OTOH, with my RoadID-I-Y, if any of my information should change, it's two minutes at my PC, 10 minutes to the neighborhood copy centre, the price of a pint of Guinness later and, voilà!, I have a reasonably durable identification document that meets my primary emergency medical information needs and is suitable for carry in a bicycle jersey pocket. Mine is about 2x4" (5x10cm), duplex printed in a 10-pt font, and is more than large enough as is, but I can make it as large the as the information I deem necessary should require.