It seems you can accomplish whatever you want in a Lookup with an Auto Enter Calc, with the exception of partial string match field (though that can be accomplished with a new relationship based on an exploding key). And reportedly, Lookups are much slower in a large solution. However, you can do a lot more with a Calc than a Lookup.

So, when would you use a Lookup instead of an Auto-enter Calculation?

This might be just a bit of legacy support.

Also the lookups permit you to make some primary decisions (do not copy when empty, etc...) without having to make a calculation.

Getting the next highest or lowest values would be difficult to make a calculation for.

Incidentally, who reported that lookups were slower in large solutions - do you have a reference?

I don't know where I got the idea from. It just is there in my head. I feel like it was a Tech Talk thread, but I could be wrong.