I'd go that far. Making the busses "free" would have a number of benefits:

1. Drivers might be in less of a hurry. MBTA busses frequently run red lights and/or sit in the middle of intersections creating gridlock. [Source: Pick any busy downtown intersection during rush hour and wait <15 minutes. Watch.]

2. In my (admittedly limited) observation, Boston bus riders tend to fall into two groups.

(a) Relatively affluent riders connecting to the T (the subway or trolley). The MBTA will still get some money.

(b) Relatively poor riders commuting solely by bus to one of the multiple jobs they're holding down per day. Consider this a kind of reverse sales tax.