some chainrings had spacers as mentioned above. However if you tried to use the chainset without them, IME it doesn't work too well; the chain catches on the big ring when running middle-small for example.
I think the spacers were or were not required depending on how the middle ring was made; if the ring has spacers it is made from thin gauge aluminium (or steel) throughout, whereas if the chainring was made from thicker gauge aluminium it would not need the spacers where it was mounted, but it would be more expensive to make the chainring because it would have to be machined thinner where the teeth are.
If you fit a non-shimano replacement ring, chances are that the spacers are not required. The spider arms are a conventional width and have not been machined, in the cranks I have seen where spacers are used.
You can clearly see the spacers (part of item #2 here)http://si.shimano.com/pdfs/ev/EV-FC-M550-1101A.pdf
in those chainsets that had them.