This is principally a precautionary measure against having the roast's outside overcook and dry out before the inside is properly warmed and cooked. And since a room-temperature roast cooks more quickly than its colder counterpart, you save energy. Of course, if the roast is 6 inches or more thick, its surface will undergo bacterial contamination before the center of the meat reaches room temperature. In that case, at least allow the interior of the meat to partially reach room temperature. An hour in the open should not pose a health threat except, for example, when the room is hot and humid.
When you cannot bring meat to room temperature, you should roast it at a slightly lower temperature than usual, counterbalanced with a slightly longer cooking time than usual. Less complicated but less effective is to maintain the original temperature but to extend the cooking time; if you remove a 6-inch-thick roast from the refrigerator and place it directly in a preheated 350 degrees F oven, add 12 to 15 minutes to the cooking time.
If the roast is frozen, increase the original cooking time by about 50 percent. Naturally, your roast will not be as tasty and juicy, nor will its exterior be as attractive, compared to a roast that was brought to room temperature before cooking.
** Oven Thermometers