Rationale:

• This was always bothering as redundant and ugly looking. Disadvantage: We now have a duplication in the ToC sources.
• The spec is a very specialized document, not suitable for the vast majority of Python users. I feel one extra click for those specialists is worth the clearer ToC for ordinary users.Edit: Especially since the spec has many chapters that will push more relevant info to the bottom of the page.
• On the guides index, directly linking to sub-chapters of specific guides didn't seem worth it to me. Users will usually want to read a complete guide.