That's a little bit of a myopic view to have. Preclinical grades aren't that important. But the only people who murder COMLEX/USMLE are in two groups: Those that did very well preclinically, and those that while not doing well preclinically, did so strategically with a nearly constant eye on those exams. In other words, they didn't really study less or study only more clinically relevant stuff, they just took the hit in grades to focus their study toward board-based material.
And I know, you're going to say why do you have to do well on boards when you can get a reasonably good residency without it? Well because if you want anything that is above-average in competitiveness, particularly allopathically, you're going to need those scores. And not all residencies are created equally.
I'm going into Radiology, and while the statistics say I'd very likely match in SOME residency if I had just average scores, the quality of residency has a lot more bearing in how your career goes than some like to admit. I'm not saying that one needs great board scores so that they end up at Johns Hopkins, but it would be nice to A- have a lot of choices where you end up. You might match your specialty, in a place you're loathe to live in, and B- tailor your residency toward YOUR career goals. Even if your career goals are meager, if you are interested in something specific, only a handful of residencies may offer that opportunity to you. FM with a lot of c-section exposure, Oncologic Interventional Radiology (my interest), OB with an ability to subpecialize in MFM (which coming from most community programs, good luck), solid tumor treatment (coming from many community programs, without a fellowship at that program, good luck). The list goes on.
The point is that scores, even for mildly competitive, or EVEN uncompetitive specialties might not be the gateway to a FIELD, but it may be the gateway to a desirable residency. It's best to do as well as you possibly can, because these grades roll over into board scores pretty easily, and then you're stuck with mediocre (if you're lucky) scores, but a desire for something just a bit greater than mediocre. But you're stuck. It's a bad place to be.