If you got a hard solution available, making this intended solution optional feels to me like actively adding in backroute options :) .
The main difference I see here is, a backroute is unintended and could be of any difficulty. Whereas with this approach, it doesn't mean I turn the level into a generic X-of-everything type. Whether this leaves one solution or several, care will still be taken to ensure the remaining solutions are of acceptable difficulty - just not as hard as the talisman would have been.
To give an example, there was one level I made for LPVI that was very hard - if you've played the LPVI demo, the level I'm talking about here was harder than Swampflower. The level in question has 40 lemmings, split between two entrances. The save requirement was 36. This was extremely hard to achieve, so I changed the save requirement to 18. But then I realised that with one extra climber, save 20 becomes possible; no real change in difficulty, but the save requirement being 20/40 looks nicer - as well as misleading the player into thinking "so I can ignore one side and save everyone on the other". Then, the talisman is - save 36, with only one climber.