And trades take up more shelf space. Not to mention they don't look right next to a Trek MM. Uneven.
Trades are made for different buyers. The fact that some collectors buy everything is secondary. Some people are only attracted to trades. They use a higher quality of paper than MMPB. The authors get extra royalty.
If you want everything to match up, Pocket should never have altered the height of MMPBs between "The Wounded Sky" and "The Trellisane Confrontration" (coincidentally the two books that came out during my first US vacation), nor published any hardcovers. My trades are interfiled with my hardcovers (in any case, the hardcovers come in three heights anyway).