There are vintages every wine lover knows to avoid. 2002 in most of Italy and the Rhone Valley was such a year and if you love classic Old World wines, you probably want to avoid 2003 in Burgundy and Alsace as well. But not everyone avoids these vintages altogether. One common advice is that if you know where to look, these vintages are a bargain buyer's paradise, as some producers will declassify their premium wines into the second wines.
Now consider the reverse strategy used to market the second wines in great vintages: Buy the second wines because the quality trickles down the food chain. You can't have it both ways so sometimes I just ignore both hypes. Basically you should buy wines and not vintages, preferably wines you've tasted yourself. But it's not always possible to tastes each wine you buy so we all depend on reviews and scores and obviously you always find high scores even in off vintages.
So why avoid a bad vintage altogether?
It's all about budget. Most of us just can't afford the risk and critics have been known to be wrong. So we avoid bad vintages as a rule of thumb, taking the risk of missing out on hidden germs.
My favorite example is the Alain Graillot, Crozes-Hermitage, 2002. It scored 93 with a local critic (who scored most of the Chapoutier Hermitage Rouges 2000's in the upper 80's, just to put this score into context) and I and quite a few of my friends each bought a bottle. I've had it 3 times and it never scored higher than an 89, and even that was the first time I had it, when I was willing to give it the benefit of a doubt. Realistically it was probably an 87 or so and had my expectations not been unduly raised, I might have enjoyed it for what it was.
By the way, if you must buy from off vintages, at least buy from vintages whose weaknesses the local winemakers have experience combatting. In most of France, you're better off buying wines from cold years (unless there were huge hails or floods) than hot years and again I refer you to 2003 in Burgundy and Alsace. In Burgundy, some vineyard practices done by rote in preparation for usual cool weather left the vines more vulnerable for the heat stress while in Alsace, producers were allowed to acidify for the first time in years and I can't imagine it being handled with sensitivity under those circumstances.