The documentation actually doesn't say
-i should be used for upgrades. It says one should use it for kernel upgrades. The reason is, that should the system fail to boot with the new kernel, you still have the old kernel available. This however is a special case - kernel packages are pretty much orthogonal in the sense that they don't own conflicting files since everything is versioned (kernel, initrd and library modules from various kernel packages can live alongside each other).
Ordinary userspace packages are rarely this independent. Should you use
rpm -i to perform an upgrade, you would likely end up with two packages of the same name (and different version) installed at the same time, with the earlier ones being broken (half overwritten). It is actually possible to install the same package twice - once that happens some tools and/or people are likely to get rather confused.