I'm seeing a lot of rationalization in your answers. Maybe I'm wrong, maybe you're just looking for excuses to get a car for them.
Well, in some ways, isn't everything a rationalization? None of the stuff we're talking about is a need. I mean, my kids don't have to go to Hebrew School 15 miles away on Thursday nights and Sundays; I didn't have to send the kids to the camp with a bus stop 10 miles away on roads that even I wouldn't bike. Hell, I could have made my kids play the flute instead of the stupidfuckingtuba so they could walk the three miles home from MS when they have an after-school activity (aside: I really, really, really wish I had made them play the damn flute). But we chose to do these things, and we continue to think they add value to our lives, and so now the question is how best to manage them.
So, sure, we carpool where we can; we limit activities; we make the kids walk/bike where it works (yes, sorry, dear, I am *not* giving you a ride to the library 1/3 mile away just because you feel lazy). But the reality is that, given the choices we have made, there are several times a week in which driving is most efficient -- especially given that we both still work full-time and are squeezing in the kid commitments around other things, like work and keeping people fed and paying bills and all the rest of that.
And my other consideration is that right now, most of that burden falls on me. And the time I spend doing all of those things is generally time away from my job, which in turn affects my ability to earn money -- just daycare pickup alone has cost me 5-10 hrs/week for the past 15 years, which either means I bill fewer hours or I work at night after the kids go to bed. On a purely financial basis, a third driver would more than pay for itself so long as I continue to work.
So, yeah, I am completely fucking thrilled that within a year, I can tell DD to go get DS from daycare so I can stay at work another hour, or to drive herself and her brother to Hebrew School so I can work or go get other errands done -- or, hell, just kick back and relax. I may even throw myself a little freedom party, just because.