To me, there is a difference between incomplete and partial answers. An incomplete answer is one, where some items from a list of otherwise identical items are left out. For example, when asked “What types of cards are in your graveyard?” and anwering “Land and Sorcery”, omitting “Creature”. With partial answers, some information is left out, but there is no expectation of an complete answer. (“What does that creature do?” - “Attack you for 2.” not mentioning the fact it can also ping creatures).
With incomplete answers, there is an expectation that a complete answer is given, otherwise why bother asking? I don't think it benefits the game at all that we allow incomplete answers. The questions to those answers are often asked to speed up play, and are usually answered truthfully. By allowing incomplete answers, we basically always force players to check for themselves, even if a player expects that the opponent knows the answer. This seems similar to hiding a card in hand when asked how many cards you have in hand, or hiding a land when asked about that. (After all, the opponent could always demand to physically count the cards himself.)
This is also not about bluffing or players being observant. In the example above, the player knows exactly what he wants to know and also knows how to come to that conclusion. Another example is a white creature that is also black due to an effect. Why should a player be punished by asking “What color is that creature?”, instead of “Is that creature black?” The former can be answered incompletely, the latter must be answered correctly if at all. But in both cases the player obviously had the right idea if he wants to attack with his black intimidater, but didn't know the intricate technicalities of wording the question correctly. Remember, that the opponent always has the option not to answer a question about derived information.
Edit: What is comes down to: By allowing incomplete answers, we promote an atmosphere of dishonesty and mistrust, even in a match where both players intend to be friendly and helpful.
(As an aside: Personally I think the Tarmogoyf case discussed in another thread is clear cut lying. Answers should always be taken in context of the question. The answer “I have a Sorcery and a Land in my graveyard.” means something different when asked “How big is your Tarmogoyf?” and “What cards are in your graveyard?”)
Edited Sebastian Rittau (April 18, 2013 11:39:08 PM)