Firstly, I agree with not adding a specific map as an example because it is generally discouraged to let a certain map be a precedent for future maps since many ranked maps have flaws that should not be taken as an example.
Also, I honestly disagree with letting such mapsets skip the spread requirements for the individual difficulties because it is indeed an easy way to avoid making a spread for those songs. If the drain time is more than 5:00 like in the case of Xexxar's map, it's fine because those diffs don't need a spread anyway. It would also work if the individual parts are longer than 4:15 and they're all Insane diffs for example. However, mapping a bunch of songs from an EP or something that are like 3 minutes long each with only Expert diffs and then putting them together in a compilation would allow you to skip Normal, Hard and Insane diffs for multiple songs which doesn't seem fair for players who want to play those songs on a difficulty that is accessible to them. This could be taken even further by making a TV size cut of each song and doing a compilation with those. Of course you can say that the marathon would be rankable by itself and this is just splitting it up into multiple parts, but that is exactly what makes the difference: If the individual songs are not long enough to skip certain difficulties, there is no reason to allow them to be ranked without a spread. The marathon on the other hand is much longer and therefore not required to have lower difficulties.
Also, I honestly disagree with letting such mapsets skip the spread requirements for the individual difficulties because it is indeed an easy way to avoid making a spread for those songs. If the drain time is more than 5:00 like in the case of Xexxar's map, it's fine because those diffs don't need a spread anyway. It would also work if the individual parts are longer than 4:15 and they're all Insane diffs for example. However, mapping a bunch of songs from an EP or something that are like 3 minutes long each with only Expert diffs and then putting them together in a compilation would allow you to skip Normal, Hard and Insane diffs for multiple songs which doesn't seem fair for players who want to play those songs on a difficulty that is accessible to them. This could be taken even further by making a TV size cut of each song and doing a compilation with those. Of course you can say that the marathon would be rankable by itself and this is just splitting it up into multiple parts, but that is exactly what makes the difference: If the individual songs are not long enough to skip certain difficulties, there is no reason to allow them to be ranked without a spread. The marathon on the other hand is much longer and therefore not required to have lower difficulties.