If your multitrack deck has individual outputs for each track, you can improve your sound and routing options during mixdown. I have a digital multitrack unit, but the built in mixer is limited. (Only 2 bands of EQ and 2 effects sends). It is CD Quality uncompressed digital. So I benefit greatly by using an outboard Mixer. I have 6 effects sends, solo capability,24 tracks to split tracks and make mono signals "stereo like" 4 Bands of EQ etc.....
So if you already have the deck.... a mixer couldn't hurt. Especially if you can use it if you add a new deck. Just don't waste money if it could be used to upgrade to a better recording device, or computer as Dragon has recomended.
Dom Franco