I do this quite a bit, though I put the day of the week in the task name, such as
"Make William's snack and lunch (Tuesday)"
which helps keep them straight. I also put on a start date so that it won't show up at times when I won't be working on it. For your Tuesday/Friday case, I might do something like a Monday start date/Tuesday due date and a Thursday start date/Friday due date for the respective tasks.
While I agree that it is a bit more of a nuisance to set up, one advantage to this scheme is that it may give you a better picture of your upcoming obligations. With your T/F example, you could see looking a week ahead that you had two such tasks in the pipeline, whereas if the implementation stayed the same as it is now and merely gave you the ability to say the task repeats on Tuesday and Friday, you would only be able to see the next one.
I haven't heard anything about a schedule for implementing more built-in options like this for either the desktop or the iPhone app. As always, send in your OmniFocus for iPhone feature requests with Send Feedback on the settings page so they are officially noted.