New Traffic Vs. BooKmarked Traffic - When Is It Better?

So i was talking to someone a few minutes ago on ICQ who told me that they have a lot of boookmark surfer traffic but all of a sudden, over the past couple of months their sales have become stagnant, they went from getting maybe 3 or 4 sales a day down to 1 sale a day and didnt understand why.

So i asked them where they were getting new traffic from and they said they didnt know, i asked if they wouldnt mind showing me their servers stats and the didnt so i had a look and discovered that 'new' traffic had pretty much stopped about 3 months ago and they appeared to be getting around 85% of bookmark traffic.

I know a lot of people tend to concentrate on building up bookmark traffic but, if thats all someone concentrates on, when does that traffic become worthless?

Your thoughts?

Regards,

Lee