Why Replace Bearings?

Here's a cautionary tale about why you should replace your bearings whenever you feel them scratching inside the hub.

Say you're rolling along and you feel a little vibration and extra drag coming from your bike. Maybe you check the bearings yourself or you take it to a professional mechanic but either way you find out that your bearings are worn out. The cost of an overhaul is $30 or more so you figure you'll just tough it out.
Well, what's happening inside the hub is that through many hours of usage one or more of your bearings has deformed and is no longer a near-perfect sphere. Its oblong shape will lead to excess friction which will  heat-up then eat-up the lubrication inside the hub. Eventually the bearing will break and get wedged under one of the other free-moving bearings like a door stopper, freezing the entire hub. This is basically the death of your wheel and quite possibly the death of you, depending on how fast you're going when the bearing fractures.

So, get those bearings overhauled if you need it and be glad.

Here's a link to an article about when to replace your bearings.