You give an example of a manufacturer's part number for a microcontroller, and embedding that number in the symbol makes sense. You place XMEGA32E5-AU on the schematic, and bang! You can generate a BOM with no post-fiddling.
But, what about resistors? Remember the real goal is to have a BOM which includes orderable part numbers, something you can submit to Mouser or DigiKey or whomever without doing any further part-number lookup. This means that a BOM which has a line
100k 1% 0805 resistor isn't useful, as you now have to go and look up a real part number for that description.
If the part number is embedded in the symbol, then you need, in your library, one symbol for each resistor value you will use. Your library can become unwieldly fast. That said, two companies I worked at did just that -- there were several company sub-libraries, one was called something like
1% 0805 resistor, and when placing parts, you had to go the library and select the right resistor value. And if you had to change the value, you ended up deleting the one resistor from the schematic and pulling one with the new value from the library.
Like I said, unwieldly, but in all fairness, it was workable since not every single possible 1% 0805 resistor was in the library.
This is where the scheme of a "family part number" can be used. But it requires some kind of post-processing that takes a partial part number (the "family") and merges it with the part value to make a final orderable part number. If you look at, say, Panasonic's part numbering scheme for resistors, it follows an obvious pattern where the only variant is the value.
Actually, it's more than that, the part number code also includes tolerance and power rating (or case size), so if you have all three of those values, your post-processing can generate the final part number.
But it all still requires that extra post-processing.
I suppose a compromise is this: you place the generic symbol RES_0805 on the schematic, and fill in value and tolerance, and after finishing the schematic, your BOM generator can go through and look for all of those things, work out the actual manufacturer part number, and keep it in the BOM generator's running list of parts. The final result is a separate BOM with quantities, part numbers, reference designators and the like, all from one operation.
This requires that you choose one vendor for your resistors and your BOM generator knows that vendor's part-number scheme.
Resistors are easy. Capacitors? Not so much.