In most cases, manufacturers recomendations are based on weight capacity as much as anything. The pressures they'll recommend aren't really "workable" numbers for many applications. That said, "too soft" is too soft. There's a practical limit for minimum AND maximum.
I've always ran between 6 and 12 psi in rears on all my tractors. Now....That's with bame brand, well made, 6 to 8 ply tires. I'd think a 4 ply "cheap-o" tire would need more psi to remain stable under a load.
All my tractors have fluid filled rears. I adjust air pressures so that I get a decent ride, the tire isn't mis-shapen under a load, and traction is close to it's maximum. There's always a "middle ground" where you can acheive the best balance of all these factors.
On my Deere 2440, I have GoodYear DynaTorque II bias ply, 16.9X30's, 6 ply. They're over 15 years old, used quite a bit, and have always been kept at/near 9psi. No sidewall failure, no abnormal wear, excellent traction (out of a tire not known for it's traction) and no issues to report.
On my Massey Ferguson 150, I've recently replaced the rear tires. The OLD set was Armstrong 14.9 x 28's, 8 ply. I ran them @ 6psi. (Heavy tire on a relatively light tractor) They lasted a little over 20 years with no abnormal wear. Now I have 13.6 x 28 Titan 6 ply "long bar/short bar". They need 8 to 9 psi to hold their shape under a load. Traction is good at that level. They're showing no wear in a year and a half of use.
Newer tractors, (Deere 6430, MF 583's) run with 10 psi. They're radial tires. The tractors are heavier and need a stiffer tire to remain stable.
You can live and die by "the book", or, you can figure out "what works". Manufacturers recomendations and owners manuals are a good starting point, but after many years and lots of hours of actual use, I've found those "rules" to be flawed for certain applications.