After you complete your Ph.D., you have two years left of med school, intern year, and then during you PGY-2, you might finally start to step back into lab for small periods of time. During these years, your research skills become rusty: you loose track of what the current discoveries are, and many techniques you use might become obsolete, and the ones that aren't you forget, or no longer do well, because you haven't done them in a few years.
You can do research without a Ph.D., some choose to earn both degrees, other choose not to, but there are successful physician-scientists in both camps. If you don't want to do both degress, look for med school with MD-with-thesis programs (most of them have them), look into research years (HHMI has two of them, NIH has a non-HHMI affiliated year program, Sarnoff, etc), also look into the possibility of taking two years off after undergrad to do research at the NIH (the postbacc program) or tech in a lab at a major med school. All of these, or a combination of them, can give you the skills you need to be an effective researcher without having to earn a Ph.D.