This question was puzzling me for quite a while.
Why is the
improvement bugfix introduced in Magento 1.13.x.x called partial re-indexing not covering "Product Attributes" and "Tags Aggregation Data" indexers? Why do these two have to be re-indexed manually?
To my opinion EAV index is one of the most critical. Not having a partial re-indexing for it means that after each sale (and some people have them) the index is invalidated which means the layered navigation is not valid anymore which results in inconsistent data display and loss of sales.
Am I missing something obvious?
So here is the description of this bug. Once the sale is made and product goes out of stock the layered navigation reflects the changes for products attributes only for simple products but not for configurables (have not tested bundles and grouped).
So let's assume in your catalog you have a configurable product like t-shirt available in different sizes and "Size" attribute is "Filterable (with results)". Then in a layered navigation of appropriate category or search results you will have the sizes section where all available sizes are listed. If after one of the sales, a particular size goes out of stock you would expect it to disappear from layered navigation. This doesn't happen unless you refresh attributes index and flush blocks cache.
This bug is really critical. It's a two way threat. First, the end customer who sees his size in a layered navigation and then sees that it is actually not available will leave your shop in frustration. Even worse thing is once the product is back in stock it will not be visible in layered navigation, so the end customer will not be able to purchase it. So it's hard to underestimate the amount of revenue losses.