In commenting on Gashimov's choice of the Schlechter Slav against Anand, I referred to that defense as "solid and slightly boring". A reader took umbrage at my comment, detecting a "hint of disapproval". In his view, Gashimov was quite wise to play something solid if boring against a higher-rated player, especially since that player was the world champion.
Leaving aside the pretty genuine possibility that the Schlechter Slav really is boring, and I wrote that because that's how I perceive it (especially as a chess fan), I think his remark confuses distinct categories. One thing we can ask about an opening is how interesting it is. Another thing we can ask is how good it is, objectively speaking. Still a third thing is how effective it's likely to be, and we can ask more fine-grained questions within this category. Let's briefly consider each of these points in turn.
Interestingness: There are various ways an opening can be (or fail to be) interesting. Just to pick out two that were relevant to my opinion, an opening can be interesting by leading to tactical complexity, or by its fecundity - that is, by being able to reach many different kinds of positions (strategic complexity). Generally speaking, the Schlechter Slav is impoverished in both respects.
Objective Goodness: This is pretty much what it sounds like. It's what we'd suppose the ultimate chess computer would play, the line that on its objective merits (rather than based on psychological tricks) gives a side the best chance to score the highest percentage in the long run. It might turn out that the objectively best opening is interesting (e.g. the Najdorf, for Black) or not.
Effectiveness: This will often have some link to objective goodness, but it need not. What matters here is what a player and his opponent are capable of. If the objectively best opening requires heavy memorization but the player has a poor memory, then such an opening wouldn't be effective. Or if the best opening in a given case is solid, but if a player loves complications and hates less dynamic ones he shouldn't use it - especially if his opponent loves solid positions.
So to affirm that Gashimov should choose a solid opening because his opponent is higher-rated is a non-sequitur. A solid line may or may not be best and it may or may not be the most effective. Gashimov might have chosen it for its surprise value, or because he genuinely believed in its value, or because his research suggested that Anand doesn't play as well in the particular types of positions likely to arise in the Schlechter Slav, or because he didn't trust his other lines, or...any number of other reasons having nothing to do with a 70-point rating gap. Indeed, there's an 80 point rating gap between him and Carlsen, but playing a solid, slightly passive line against a player like Carlsen who is happy to take a small advantage and play forever is probably a very bad decision.
The moral of the story: when deciding what to play against a higher-rated player - or against anyone, really - solidity is not an end in itself. (Unless your usual openings include rubbishy stuff like the Latvian and Englund Gambits!) Play first to your strengths, second to your opponent's weaknesses, and within this context choose the objectively best opening that fits the bill.