Is there a benefit using SPD SH-51 cleats over SH-56?
SH56 releases when you pull up on the cleats, ironically defeating the largest benefit you’re trying to achieve by using a clipless pedal system. Using SH51 allows you to benefit from that upstroke.
Before anyone tells me that “you can’t pull up!”, try pulling up more than you push down while sprinting  out of the saddle, clipped in. You can actually pick the entire rear end of the bike up off the ground with each leg by pulling up hard enough. Try doing that with flats!
As you said, using SH56 will make it easier to remove the shoe from the pedal, defeating another purpose of clipless systems: better foot retention. Racking up the pedal tension while using SH56 in an effort to make release more difficult defeats the purpose of SH56.
SH56 is a crutch for inexperienced clipless users who haven’t mastered the clip-out movement yet. That’s it. Arguably, it is best to never use it so you completely remove the pull-up-to-release instinct from your body. Simply turn the tension down as a beginner. You’ll pop out in a crash, don’t you worry.