I recently had a discussion about this with some of the members of my group.
It's very important to give out XP based on the individual CR of the creature.
Looking at the table in the DMG, you will find that if you group monsters into EL and then use that to give out XP, it will be (approximately) the same as when you multiply the single monster XP with the number of monsters. (approximately, because some EL's are given for 7 or 8 creatures, etc.)
Why then is it important to use the individual monster CR for XP?
Because below a certain CR, monsters don't provide XP anymore.
The reasoning behind this is that starting at a certain point, low CR creatures can't hit you, can't damage you, and are probably obliterated within the first round without costing any resources.
Using a lot of these creatures would increase the EL, maybe even up to a point that it would yield XP if looked up in the XP table as 'effective CR'. However, the combined forces of those opponents still can't hit you, can't damage you, and will be obliterated in a few more rounds, still without costing any resources.
Bad PC! no Xp for you.......
Herzog