For lower levels... you take what you can get... and there is a limited amount of gear available to you. As the other guys have mentioned... 1 point of strength yields 1 attack power. Agility is superior because it adds crit and dodge as well as the same amount of attack power per point. You may find yourself wearing gear that has strength on it simply because it was available and an upgrade over what you had. At higher levels this isn't really true (especially in the long run). "rogue" armor is typically designed with some combination of agility, attack power, crit rating, and hit rating (and of course stamina... which is true of all classes). Leather armor that has strength on it is typically targeted at feral druids. There is enough rogue specific armor available at higher levels that you won't really find yourself wearing anything with strength on it.
Also, as far as itemization budget is concerned, 1str = 1agi = 2ap. That is to say that if a you have 3 pieces of armor of equal quality, they could have x str, x agi, or 2x AP on them (all other things equal). Obviously, if you are getting only 1 ap per point of strength, you would be better off using gear that has agi or AP on it (giving you more total damage).
Edited, Mar 17th 2008 10:41pm by LordMeridus