Stan is right. Really don't need more than 10 rounds (we hope). That being said, the internal magazine is cleaner, can't get dinged and cause misfeeds, allows for easy top-off (or bottom-off if you wish), is always with the rifle, etc. Also correct that stripper clip reloads can be as fast or faster than magazine change.
I think the Russians were smart - designed a foolproff and reliable system. Magazines became necessary on AK in part because of full-auto fire, which is not a consideration with SKS.