that is only true if you wear pants with different rises on different point of your waist. i wear all my pants on the same point of my waist, with maybe a variance of 1/4 inch. so in my case, the outseam is fixed. a variance in inseam will only affect how much the crotch drops/sags, for me. ask clapey, he will tell you all about it!
That is the main (though not the only) point of rise measurement - to define where on your waist they're going to sit. Ideally, the amount of sag should be the same on all trousers (as much as you prefer for comfort), and if you like them to sit on the same point of your waist they should always have the same rise. I tolerate a wide range of "waist positions", but if a RTW pair has too high of a rise even for me, I just won't buy it, rather than have it sag, which I think looks sloppy. Once you have found your perfect measurements, both inseam and outseam should stay fixed. Outseam/inseam balance is also useful to correct for bow leggedness or knock knees...
BTW, I'm not implying you have this problem on your last fit - I think a lower button stance on the jacket would help more than less sagging on the trousers, but that's a different issue.
Edited by RDiaz - 8/27/13 at 3:02pm