FWIW, having myself a box featuring piix4_smbus which would not even boot with TC stock kernel without 'acpi=off', the major functionalities I miss are temperature and software battery indication [hardware battery indication being less detailed]. cpufreq scaling seems not to be affected.
Same box would boot with acpi in the past with Linux 2.6.16 and a different distro, but buggy behavior throughout operation led me to prefer apm.