... if you have a bunch of favourite songs below average track duration, their rating will not get scaled down, which can be a desirable result to you.
This is a very desirable result... I ran two 'Adjust Ratings' back to back, one with the duration enabled and one disabled. Here's a snippet of the log with the variances (the formatting's not great but you get the point. )
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* 2008-02-01 15:30:55
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Score with Duration Included (duration) vs. Score without Duration Included
101 for The White Stripes - Ball and Biscuit (7:16) vs. 99
095 for Boston - Carry On My Wayward Son (5:21) vs. 87
095 for Billy Joel - Piano Man (5:36) vs. 86
091 for The White Stripes - Dead Leaves and the Dirty Ground (3:04) vs. 99
091 for Carbon Leaf - Let Your Troubles Roll By (5:26) vs. 84
090 for Yeah Yeah Yeahs - Bang (3:09) vs. 97
090 for The White Stripes - The Air Near My Fingers (3:39) vs. 92
090 for Norman Greenbaum - Spirit In The Sky (4:02) vs. 90
088 for Yeah Yeah Yeahs - Gold Lion (3:09) vs. 94
088 for Remy Zero - Hollow (6:20) vs. 79
088 for Allman Brothers Band - Ramblin' Man (4:42) vs. 85
087 for The Crystal Method - Bad Stone (5:09) vs. 80
086 for The White Stripes - Icky Thump (4:18) vs. 84
086 for The All American Rejects - It Ends Tonight (4:05) vs. 85
085 for Kid Rock - Only God Knows Why (5:27) vs. 79
085 for Billy Joel - Oh What a Night (3:36) vs. 87
084 for Styx - Come Sail Away (6:04) vs. 77
083 for Whitesnake - Here I Go Again (4:37) vs. 80
083 for Under The Influence Of Giants - Mama's Room (3:47) vs. 84
082 for Van Halen - Panama (3:32) vs. 84
082 for Tom Petty - You Don't Know How It Feels (4:50) vs. 79
082 for The Jimi Hendrix Experience - All Along the Watchtower (4:01) vs. 82
081 for Soundgarden - Black Hole Sun (5:17) vs. 77
081 for Outkast - B.O.B. (5:04) vs. 77
081 for Four Tet - My Angel Rocks Back and Forth (5:07) vs. 77
080 for Ulrich Schnauss - Knuddelmaus (7:01) vs. 68
080 for The Killers - All These Things That I've Done (5:02) vs. 77
080 for Carbon Leaf - Life Less Ordinary (3:34) vs. 82
079 for Yeah Yeah Yeahs - Cold Light (2:16) vs. 89
...snip...
PlayedCount: 35%, SkippedCount: 0%, LastPlayed: 0%, DateAdded: 65%
Maximum Played Count: 14, Maximum Track Duration: 1196, Average Track Duration: 236.466846569005
Overall it seems like (as you suspected) the duration factor was causing my shorter 'favorite' songs to lose a bit of their rating, and bumping a few longer songs higher than I feel they should be. I think I prefer it without the duration being a factor... Thanks for implementing the option!!
Ryan