Why decant?

Last night, the "lovely young lady" asked me when one should decant a wine (I will write another post at a later date about why the question was raised). There's one, clearly accepted reason for decanting wine, and there is another, more controversial reason to decant.

First, some wines are likely to have sediment. Vintage Ports, for example, are bottle aged with the skin fragments of the wine grapes in the bottle. These fragments can be substantial in size and, take it from me, one does not want to consume them. Other wines, as they age, will throw down tartrates which are harmless, but aren't pleasant to look at in one's glass.

The second reason some people argue a wine should be decanted is to aerate it by exposing all of the wine to oxygen. On the other hand some researchers have argued that decanting actually has negative effects on wine (or at least, some varietals). I, honestly, fall in the former category for certain types of wines (young, powerful wines). After all, when one wants to open a bottle... why should a little thing like youth get in the way?