It's interesting Mass General didn't recommend mop-up chemo. Medical journal articles frequently quote the statistic below (J Med Life. 2015; 8: 12–14):
Nevertheless, tumor recurrence after curative resection of colorectal liver metastasis remains a major problem. 50% to 75% of the patients develop a recurrence of the disease within two years.
With those kind of odds, it just seem prudent to do adjuvant chemo. However, some oncs do a less stringent follow-up for various reasons: advanced age of the patient, severe comorbidities, or what I suspect happened in your husband's case - favorable genetic test results (KRAS wild, normal MLH1, MSH2, MSH6, PMS2, etc.) That isn't the safest approach, but their thinking is that it prevents the patient from unneeded chemo. It's a gamble though, anyway you look at it.
In my case, the prognosis was pretty grim. A recent population-based study (BMC Cancer. 2018; 18: 78) involving 1,026 patients (so not a small study) came up with the following statistic.
Patients without any metastatic disease had a 5-year OS of 75.1% compared to 25.2%, 45.7% and 12.7% respectively for patients with liver-only metastases, lung only metastases, and liver and lung metastases combined.
That puts me in the 12.7% category. I know MSK and the HAI pump are what made the difference in my (hopefully) long term survival. Things are fairly simple when you're a low stage, but treatment for Stage 4's is all over the map, unfortunately. My advice: Do your research, decide what's best, and then find an oncologist/cancer center who is in accord with your thinking.