Product review rating

Why do you often give product reviews a mark of say 4.5? Is this due to only rating products out of 5? And the differences between 0 and 5 not being enough.
Would it be more logical to mark out of 10. So 0.5/5 = 1/10, 3.5/5 = 7/10 and 5/5 = 10/10.
Would it be more logical to mark out of 10. So 0.5/5 = 1/10, 3.5/5 = 7/10 and 5/5 = 10/10.
Planet X Kaffenback 2
Giant Trance X2
Genesis High Latitude 2x10
Planet X n2a
Genesis Core 20
Giant Trance X2
Genesis High Latitude 2x10
Planet X n2a
Genesis Core 20
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I think the reasoning is that many people relate to a "5 star" review more - indeed they are marked out of 5.
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Thanks for your replies. I can understand the rationale of scoring out of 5, however as Jeff states, half marks are awarded too often. So really, a change your half mark policy should be considered, or mark out of 10.
Giant Trance X2
Genesis High Latitude 2x10
Planet X n2a
Genesis Core 20