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Here is the thing streaming would be a nice option, you you are on a good network connection, if you have unlimited data, and if you have to much music for your iPhone. For the normal user, none of these situations probably exist. For the average Macrumors user these situation might exist, but for the most part streaming is an unneeded option.
The only thing I wish they did was have so that you could create a playlist on iCloud of music you want in the list. Then instead of downloading individual songs or albumbs, you could hit download playlist and every song on that playlist would start downloading in the background. You could start this in morning at home on wifi and by the time you are ready to leave for work or school most of the songs should be downloaded leaving only a small amount to be downloaded over 3G against you data cap.