Also, the NHS route receives far more daily traffic than that segment of CT-66.

Route 32 between 66 and 6: 14700 AADT
Route 6 between 66 and 32: 7200 westbound, 14100 eastbound
Route 66 between 6 and 32: 6100

This shows that the CT-66 local route is actually used far less than the CT-32/US-6 bypass route. This increases my opinion that even though that part CT-66 feels like a main road to Willimantic townies (formerly myself included), highway=secondary is more appropriate by multiple metrics