I don't think it's a waste of 6 seconds, it adds weight to the gameplay and puts you more in tune with the games physics and rules - the spindash basically breaks those rules and disconnects the player from the reality of the game world.
Watch any sonic 2 speed run - spamming the spindash is not in the spirit of the original game philosophy.
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I won't argue that Labyrinth and Marble were totally inappropriate and clash with the design
I would argue they aren't inappropriate. Each zone in Sonic 1 is perfectly designed with a deliberate intention in mind.
Green Hill - Instant crowd pleaser, easy, introduces the curved surface and ball rolling elements early, but almost in an "on-rails" way (eg the "S" tunnel)
Marble - Back to traditional platform design, blocky grid patterns, traditional spike and lava hazards. PLayer is more familiar with this design from other games (ie Mario), but it will feel a bit different with the fluidity of sonic's movement and the taste of what was to come offered up by green hill
Spring Yard - Slowly introducing curved surfaces in a hazardous way (unlike the fun curves in Green Hill). Still retains some traditional blocky/grid elements, but with a twist (moving blocks). Half pipes are an obvious introduction to gaining momentum. PLayers may struggle to rise to the challenge on first play through.
Labyrinth - Back to a more traditional platform design to tap back into player expectations and prior experience - challenges the player by removing sonic's speed and adding the risk of drowning
Star Light - Brings back elements of Green Hill zone's design, but with more threat and challenge, to once again test the players ability to cope with curved surfaces and managing momentum. Bottomless pits feature as the primary obstacle.
Scrap Brain - ties all of the previous elements together - testing the players abilities that have been learned and honed along the way.