Just as a quick theory here, but I'm wondering if the spindash is partially a space-saving device for levels, if you get my meaning. As in, since it may require a run up to get over a loop-de-loop, that would take space up for adding other hazards and items to the level - which would make getting around the loops more tricky had there been another way to go over them. So maybe the spindash enabled the developers to add more stuff to levels - and to enable more interesting terrain (as what BounceDeLemos stated) - all the while having larger levels as well. However, that's probably more likely why springs and boost pads etc. were positioned just before loops in later games which took out the need for the spindash; to the point where it's become almost like an interactive movie. It'a not the spindash I have a problem with, but the boost pads and automatic sections which are what's ruining the modern games - as we very well know (ignoring Sonic's own Boost mechanic - which many/some people like).