I agree that he is by far the best position player on the Mariners. Here are the reasons why he should bat first:
1) Typically, when you have a player who hits for a high average and has a high on base percentage, you want him to get as many at bats as he can. The lead off spot usually gets roughly 60-70 extra at bats a year than the clean up spot.
2) You want run producers to get ahead of the hitters who drive in runs. Think this: Carl Crawford (who does have some pop) is a fantastic lead off hitter because of his high average and OBP. What makes him scary is that he can take second at will.
If you lead the game off with a walk or single and then steal single, your automatically in scoring position. The second hitter in the order is usually a player who can move the ball around well (Placido Polanco is considering a terrific second position hitter because of his ability to move a runner from second to third). You now have one out and a runner on third.
Ichiro is very similar to Crawford, but gets on base more and hits for less power. If you can get him 60 extra at bats a year, you want him to (that's roughly 22 extra hits for him a year).
If Seattle had productive hitters in the 3/4 holes, Ichiro would be one of the most valuable players in baseball. In 2001, Ichiro was a monster because he had Edgar Martinez, Bret Boone, Carlos Guillen, and Mike Cameron in the line up with him.
That help a little or?