An interesting question I agree. Fortunately, in my experience, many PI's would kill to have an MD/PhD student work in their lab. I don't mean to sound egotistical or high & mighty but I will say that typically the average MD/PhD student has more research experience and is more qualified than the average PhD student. Again, note the use of the word "average." This is indicative of the competitiveness surrounding the process to become admitted into a PhD program vs. MD/PhD program. I think people will agree in that admission into the latter is much tougher than the former and selects for the some of the best candidates.
MD/PhD students, in addition to being very qualified, also have time pressures motivating them to be productive. The MD/PhD student's
modus operandi is to get the work done, graduate, and GTFO quickly. In the long run, the PI really cares about productivity in the lab. A high level of productivity allows the PI to obtain more grant funding, get previous grants renewed, advance along the tenure track, etc. Hence, these motivations from the perspective of the PI as well as the student converge quite nicely.
Regardless, there are a few bosses out there who really take advantage of the fact that graduate students (MD/PhD or PhD) are cheap labor; they will want to keep the student as long as possible even if the student is super-productive. It is your job to avoid these mentors at all costs

. OK maybe that's an overstatement but you get my gist. In a perfect world, you will choose a lab that does interesting work, that is headed by a supportive PI who provides good mentorship, and most importantly where you will learn how to do GOOD SCIENCE...and will get you back into the clinic/wards in a optimal # of years
🙂
In summary, my experience tells me that MD/PhD students are in high demand. When I visited my top choice schools for a second visit after being admitted, I met with many PI's. Many of them were very eager to have me as a student--including PI's that typically hire only postdocs in their lab! As an MD/PhD student, I am confident that you will be highly sought after and that there will be mechanisms to help you succeed in your endeavors.
Happy Thanksgiving!