You don't know what the end spell will look like though, for example with rank 7 of the rune it may have a 100% chance to go through the first 4 targets. Additionally magic missile may very well gain missiles as you level it up, meaning you're shooting several bone spears @ once. So it would still be effective in that case because of your 8 missiles you shot maybe 6went through the first enemy 4 through the 2nd, 2 through the 3rd, 1 through the 4th and 5th.
Which is why I revised my statement in the third post. Sheesh... Why do people feel the need to knee-jerk-respond to the first post they see that they don't like?