Maybe part of why folks are not tuning out during the spot breaks is, those shows air during the day while people are at work, not in their cars. My guess is, the folks in their cars may jump around the dial during spot breaks, as I do. If I'm listening to talk on WDEL and they go to a spot break, I'll jump to WNTP, WPHT, WILM, or WHYY to hear what they're airing. Sometimes I make it back to WDEL, other times I found something of more interest at one of the others and stay there until their next spot break. That is where WHYY-FM has a real advantage as they're breaks are like 30-60 seconds total, so I don't switch when they do those breaks as they're short and not irritating as so many commercials are, for my ear.
When at work, folks are busy and the talk is on in the background, and maybe they can't get to the radio to change it, or there's more than one person listening in a group setting, etc, or they're only partially paying attention as they are busy working and they just ignore the spots, etc. Listening at work is different from listening at home or in your car.
My guess as to why station's like B-101 (WJBR for the Wilmington market) do so well during the work day is they are the "safe" stations for an office or work place to choose. Nothing too wild, too loud, or offending like a Rap, a heavy metal, or even a Country station might be. Kind of modern day "Muzak", except this muzak does appeal to the younger female demo the advertisers crave.
Talk radio really is more geared for individuals who have their own office with a door that closes, so you're not offending the others who won't want to hear the rantings of Rush/Beck/Hannity, or even the verbiage of a local talker on WNTP, WPHT or WDEL, be they lib or conservative as religion and politics are two real no no's in the workplace. That might also be an advantage for WHYY-FM (NPR) as their talk is calm and not loud or offensive, etc, which also makes it a safe choice for the workplace.
I've noticed on WDEL that the only callers they generally get on their live and local shows are either retired or unemployed folks as most people at work, don't have the time to hang on the phone for 15 + minutes to offer a 30 second comment (way longer if calling a national show like Rush).