With IFS the front wheels move up and down independantly of each other allowing a bump/dip to be accounted for at the end of the axle that encounters it. This means that the wheels are flat on the road and bump/dip at the same time.
With a solid axle when one wheel hits a bump/dip, because both wheels react together, they both run along their edges. Also because both wheels are 'tied' together, if one wheel bounces then the other has to follow suit.
I hope that makes some sense....... It makes in my head anyway

(there again strange things go on in my head

)
Ian