If you are going in with a long drill (x6D or more) it's usually recommended to use a shorter drill to create a pilot hole. However for 5xD and shorter you don't need anything. Though this can change per manufacturer recommendations.
I think he's referring to a U-drill and not a letter U drill. If it's a letter U then he should clarify and yes you would want a spot unless it was a screw machine length or carbide drill.
Even with two inserts, they're effectively single flute and wouldn't follow a spotted hole. The center insert is usually designed to guide it in straight.
If you are going in with a long drill (x6D or more) it's usually recommended to use a shorter drill to create a pilot hole. However for 5xD and shorter you don't need anything. Though this can change per manufacturer recommendations.
If you have a long drill and don't want to spend money on a second, shorter drill, you can use an endmill and helical bore a starter hole about 1/2" deep.
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