Keeping the indoor %RH below 50%RH is important to avoid mold and dust mites. Erv/HRVs are unable to keep the home dry during cool, damp weather. Opening the windows can also make your home damp enough for dust mites to grow.
I agree with fresh air when the home is occupied and maintain <50%RH all of the time. You will need about 90 cfm of fresh when the home is occupied. With the mild weather, you not get much benefit from and ERV and will also need dehumidifier during low/no cooling loads. Much more practical to get a ventilating dehumidifier. I work for ULtra-Aire/Therma-Stor. We sell both ERVS and dehumidifiers. I agree with your a/c contractors. No pay back on the ERV and the make-up air fresh helps the bath fans and clothes drier to operate. Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"