I'll take the opposite position, then.

While the intention of backgrounds is to incline towards roleplaying more than optimization, they do offer mechanical benefits in the way of extra skill, tool or language proficiencies. It's true that there is more leeway in how the system works, in that you can replace proficiencies, but this is limited to DM allowance. After all, it is extremely strange to have a Sailor gain proficiency in the Arcana skill, since it makes little sense in doing so (Nature would make more sense), or a Soldier to have proficiency in thieves' tools unless it also happened to be a street kid, in which case an Urchin makes more sense. Thus, you have to take the backgrounds as-is when making a build in the first place, and then cover for the alternatives: if the DM allows it, you could make a change, but if not, you need to have a fallback. This is a choice, and optimization guides aid readers in making conscious choices.

Think about it: if you're telling people which skill proficiencies are (or aren't) good, you're telling them the conscious choices. If the reader has an idea in mind that it wants to develop, though, it may find that recommendation to go against its wishes, and it'll ignore it. Leaving backgrounds undefined may cause people to make similar choices; if it's part of the build in terms of roleplay, no optimization guide will change your mind, but if it's the opposite (background is considered less for the roleplaying aspect and more for the optimization aspect), the reader will want a conscious judgment of each choice. Backgrounds can help pure optimizers to tap into their roleplaying aspect by giving them what they want to have the edge, and yet give them random ways to build their character. Note that Background Analysis stops at the roleplaying trait; it doesn't analyze the Personality Traits, Ideals, Bonds and Flaws because all of them are irrelevant to optimization. Proficiencies, however, are.

Thus, an analysis of all backgrounds in terms of how they are presented is necessary. Guides are made without houserules in mind, after all; assuming that because background aspects can be explicitly replaced doesn't mean it'll be the norm. Organized Play would be such an example, where Backgrounds may not have the same degree of alterations than on a particular house table.