Haki requires a certain amount of energy to use. Since Haki refers to the will power of the user, it uses mainly mental power. Though we all know that thinking too much for a long period or intensively thinking about things, makes you tired and thus drains your energy.
It seems that Observation Haki cannot be turned off and is the odd one of the three in that sense (Eg: Enel knows what everybody says at any time, Aisa and Cobi kept hearing people's, driving them insane at first). Here, it does not costs you energy to turn the ability on, but rather to keep yourself sane hearing so much voices at the same time for all time. It must have been so intense, that it would have been impossible for Whitebeard (due to his illness) to concentrate on everybody around him during the Marineford War, giving Squard the opening to stab him. As mentioned by Crocodile and Marco, he should have been able to sense and avoid that attack.
Releasing Conqueror's Haki requires concentration, making it a bad move to use excessively. It would only drain precious energy and wouldn't really effect stronger opponent anyway, so unless you want to intimidate or maintain a badass image like Shanks it wouldn't make much sense to continuously release it.
Finally, Armanent Haki requires the most concentration and energy of all forms of Haki. Coating for a long time or coating large surfaces takes a major toll on the mind and body. The user might get used to using such amounts of Haki over a long time, but it would drain their energy pretty rapidly.
Luffy fully coated his body but was completely worn out after doing so for only 20 minutes. Additionally, using such amounts of Haki took such a huge toll on his body, that Luffy was rendered unable to even stand and needs at least 10 minutes to regenerate his Haki, before attempting to do anything else again.