We are just entering the same discussions re taking bikes on our trip. We've come to the same conclusions re biking in Europe, i.e. there are plenty of great bike routes and cycling round towns would be more fun than in a landy.
In Africa I've had a bike in a series of places, Somalia, Zimbabwe, Ethiopia and Mozambique and generally it's been great fun.
Obviously it's a rare traveller who'll be taking anything to Somalia and I just used mine whilst working there. Ethiopia was difficult as kids threw alot of stones and were generally a pain in the arse (and I was working for Save the kids then!!!), but in Moz and Zim, it was fantastic and a great way to be less obtrusive in visiting a country.
The low impact arguement is thus a strong one, it's a very different introduction cycling in to a village, than turning up in a fully laden 4 x 4.
But also for us it's also an opportunity to guarantee some decent exercise. I've done quite a few car journeys for work in Africa, but this is my first private long haul trip by car, and one of my fears is ending up with a huge drivers curve, front and back...that is more than I already have.
To be honest I'm looking forward to parking up the bike and, for example taking off for a few days and cycling along the R. Niger, and then possibly taking a ferry back to pick up the car.
Anyway, good luck.