If the kid really really enjoys doing experiments, this is a good combination. If the family does not want to pay for the medical school, this is a good combination; i know many American people have been doing this. For me, the extra 4 years of phd work does not worth it. A md can do really good research; he/she does not need to be a phd to do research. To my knowledge, NIH has lots of special funds to encourage a md to do research. A MD can simply hire a good phd to run the lab for him. If a doctor, md or md/phd decides to do research, in many cases he will earn less, because he will seen less patients, which is translated into less income. For a doctor to start to be paid, he usually is at mid-30s. Adding another 4-year does not sound good to most people. So my suggestion is that if one can afford to go medical school, md/phd is not a good combination.
To my best knowledge, for any decent md/phd program, usually it will take at least 4 years to get the phd part done. Seldom one can get it done within 3 years.