John 21:25 does not say anything against sola scriptura which maintains that everything necessary unto salvation is contained in Scripture. The position of sola scriptura is not to eliminate the importance of tradition in the church, but to keep the church from appealing to tradition as authoritative OVER Scripture. Again, this is what creates problems when we try to debate certain doctrines because when one argues that a certain doctrine is unscriptural, the response is to point to tradition as an authoritative proof that the doctrine is correct. When I challenge the position of tradition as being authoritative to that extent, you point to Scripture!
I think that tradition is something that a lot of the Protestant world has cast aside to its detriment. But I maintain that if Scripture and tradition are at odds, Scripture must necessarily win out EVERY SINGLE TIME.