May as well. A runner at second with zero outs is worth 0.6 runs according to baseball-reference. I assume this means the assumption is the runner will score 60% of the time. If you are the defending team, you may as well increase your chances to get a double play, at minimum, to counter that. By putting another runner at first, the 60% already counting against you probably only goes up to 70% or something, whereas your chances of getting a double play would no doubt go up much further than 10%. And, of course, it makes a triple play possible. This rule is also not fair at a league level. Meaning, the NL will have a 2-in-9 chance to either get the pitching position at the plate or be able to walk someone to get the pitching position at the plate. In those cases, the baserunner might be less valuable.