I used my on board Realtek for a while and I must say that to me it wasn't that bad although I fell better whit my SB Audigy SE

(specially when I have daniel_k drivers

). I bought my Audigy when I was still on my old system witch had a on board C-Media card whit 2 channels and at that moment I needed a 6 channel card coz I got 5.1 speakers from my friend for free

and the card was so old that it started to give me more shitty sound than before so I changed to Creative.
For better performance you should have X-Fi that Fatal1ty Pro gamer or something like that version coz it has that X-Ram I guess. If you don't even hear the difference in sound there is no need and you feel fine whit that on board then for you there in need to use another card. And if you need other slot for like TV tuner, USB/FireWire hub, gameport

, any kind of controllers or whatever it will simply take more space on your mobo and in your system.
In conclusion it simply depends on your need.