AV in is also useful if you want to record your finished edited movie back to DV tape for keeping long term. DVD's have a limited life compared to tape. If you record back to tape, you have a full quality original of your movie that can simply be transferred to any computer again in the future, should you wish to re-edit or make more DVD copies, or played back at full quality to a tv etc. If stored correctly, they will outlast DVD's by a large margin for keeping your movies longer term.
I too looked at the 450 vs 460 but hesitated as I felt the AV in function would be useful. I admit the price is VERY tempting though, especially if you get a 'quiet' camera