I think if riffs where repeated in different keys you would not have as much different material.
Allowing transpositions gives a lot more. That's what's good about Alt+F8 being able to create your riffs bar by bar or note by note. If you disable transitions this will allow a better range of pitch shifting the whole track.

This allows transpositions, some are +- 2 semitones:
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This is with transpositions disabled:
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