^Not that he's Everyman, but my father would be one example of the hypothetical man you refer to. He drives to work at 1 Beacon every day from Assonet. Because of bad experiences on buses to/from the area, he laughed at the idea of the buses/ dedicated bus lanes. This is the same guy who is chomping at the bit to be able to ride a CR train from a Freetown station simply because (of the perception that) it's a lot more reliable.
The reason I say it's hard to promise the same consistency as rail travel is simply because I'm not sure where the bus lanes are going to go/ who they're going to be open to. I'll give this the benefit of the doubt and say that they'll extend from exit 12 on 24 (the 24/140 interchange) all the way to the tunnel entrance or S. Station exit on 93. Even then, do you restrict those lanes to MBTA buses (assuming the MBTA operates them as an alternative to the train), or do you open them to private buses like Peter Pan, Brockton-Plymouth, etc? and when you get to 128 and 93 do private buses from the Providence area, Fall River/NB area, Plymouth/Rt. 3 area use them? If so, there's always the risk of back ups of buses, breakdowns, etc.
If there is a breakdown or hickup in service, how do you bus people around the accidents or problems? you can't unload them from the middle of highway lanes onto different buses to continue the commute. With rail, you bus from station to station around the problem.
Next, given that the commuter rail will have stops outside of Fall River and New Bedford, how do the buses leave their lanes to pick up passengers to pick up passengers in Taunton and other stops at points North of the terminus stops? Fall River and NB are urban areas (and to an extent, so is Taunton); as a result, buses are subject to the same traffic signals, auto traffic, construction, delays, etc while off of the highway in these areas as anyone else.
Capacity is another issue. Fall River/ New Bedford is a populated (and growing) area. It's hard to serve the same portion of the population with a bus as you can with Rail. It would require a bunch of buses, back to back, to do the same amount of work as one train can. This increases the chances for incidents and also increases operating costs. Most commuters are guaranteed a spot (either standing or sitting) on say, the hypothetical 7:30 train. If you're aiming for the hypothetical 7:30 bus and you don't show up early, there's a good chance you're not getting a seat and will have to wait for whatever amount of time it takes for the next bus to leave (I'd assume buses would be more frequent than trains, but it's more time regardless).
Comfort is a question that I worry about too. If there are hard plastic seats like the horrific bus trips Ron refers to, then I'd be willing to bet people would rather drive.
I have a bias as a result of growing up on the South Coast that plays a role in my enthusiasm and expectations for this project. Even putting that bias aside, and acknowledging that dedicated bus lanes would draw SOME ridership, I really can't see them as a good alternative to commuter rail travel.