You guys must realize that putting Banjo in SSB isn't just "free advertising" for Microsoft. It's a proving ground, a field test that shows people are still interested in the character on Nintendo systems. In turn, the popularity (or not) of the character may determine future titles starring/featuring said character... a straw-poll of sorts: "Are people still interested in Banjo-Kazooie? give Nintendo $3 to vote 'yes'!"
The same goes for for similar characters/IP's, owned by other companies but iconic to Nintendo consoles, such as Simon/Trevor/Richter Belmont (Castlevania), Travis Touchdown (No More Heroes), Terra Bradford and/or Cecil Harvey (Final Fantasy), Isaac/Felix (Golden Sun), etc., etc.