There are situations where people are away from shore power for weeks or months, aren't regularly getting the bank back to full, or are too often drawing below 50%.
Knowing, really knowing with accuracy their SoC is then critical to getting the system to where they'll get 12-15 years from that multi-thousands bank rather than murdering it in 3-5 years.
For that kind of scenario voltage alone just doesn't cut it, and even with SG one should get the SoC correspondence chart from the maker.
But I do realize this is more rare in the world of trawlers than cruising sailors or even expedition/RV/caravans.
All right, all right!! You are surely true in your statements that it is another good way to check battery condition via hydrometer readings of SG and Soc when on long voyage or anchored for weeks/months. But, please don't forget... in many boats it is a contortion exercise to get into hydrometer check-position for every cell.
Therefore, on a multi year old battery: Will checking SG or SoC actually expand it's life span any more than simply using a multi meter through wired into a comfortable to access cabin placement position? So by flick of a switch you can make sure to not drain batts down past 50% level and also make sure that batts are getting charged amply to reach 100% charge??
Seems to me that LA deep cycle batts just need to be kept filled with distilled water, not let go down past 50% charge and recharged most times to 100%. That procedure gives LA batts the best chance for long life. If a batt gets old enough to where it won't hold a charge and/or it too quickly discharges then that is called a "Bad-Batt".
Of course, once a batt has seen its day pass and it simply has run the course after years of service then it is time to replace. Also, IMO - If after years of service [ours is 8 yrs. old] considering you have a multi batt parallel run batt bank [we have four G31 LA, deep cycle] for house power or anything else [our batt bank also starts engines] and one or two batts give up the ghost, i.e, become "bad-batts"]... then it is time to replace all the batts. Reason is: Even if a couple of the many year old batts still seem OK you can be sure that in near future they too will go bad and drag down/ruin the couple of new ones recently installed.
Batteries on a boat are important of the overall power package. Boat batts and all the many levels of batt equipment can become very expensive [multi thousands] or kept to a basic minimum [under a thousand].
The batt system complications and expense as well as living comfort and energy service availability of batts on a boat is at the sole desecration of boat owner. Research is necessary for any boat owner to arrive at the batt-package they want aboard their boat. God knows there are enough links on Google as well as threads in forums to satisfy all inquiries about marine battery power.
Happy Batt Daze! - Art