The reason for this is your not doing the "wipe data/facory reset" which will keep all your apps etc.
you should always wipe data/factory reset when switching between roms.
If your going from cm7 nightlie 80 to cm7 nightlie 81 then no you wouldnt need to do this.
I wipe data/factory reset and wipe cache partition (well i personally wipe them twice) when flashing roms.
HTC Incredible 2 S-off
HTC Droid Eris
CyanogenMOD 7.1.0 RC1 Gingerbread NIGHTLY