lurker in read only mode for 2 years b4 asking a question (i hate to ask what has already been answered)...

i have read posts about how the only way to get UAL miles from MR points is to use a hotel program as a conduit (at lousy conversion rates). my confusion:

you can convert MR to US Air and US Air is part of the star alliance- obviously including UAL. so... why don't people do that? i have 70,000 MR points i plan to transfer to delta today with the 35% bonus (already confirmed with them that my registration is valid for the 35%). but, being in sfo, the UAL is more usable for me and, from my experience, one heck of a lot easier to find a seat on for both domestic and intl. so, if i can fly UAL thru transfering to US Air, that is what i'd do.

can someone explain what i'm missing (ie: is it that US Air's % of UAL seats is so low that you can never get a seat?). like i said, i'm confused.

thanks!