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Performance. If you have (+ x y) and you can infer that x and y are fixints, then you can use a couple inline opcodes, rather than calling GENERIC-+.

EDIT: lispm is right, too, of course. You can look at it either way. If your program required top performance and you were going to tag every type until it ran super fast, then inference buys you the convenience of not needing to do as much. If you weren't going to bother with that, then inference buys you extra performance without all the effort.