re the stabilising function mentioned by Giropaul; yes you shouldn't use two-bolt cleats on shoes without a sole recess unless you also have 'pontoon' like stabilisers or similar. Shimano (and others) don't make this adequately clear when they say some shoes are both SPD SL and SPD compatible, but they definitely mean them to be used, not just for SPD cleats to be screwed on directly to the sole so they stick out.
However it is very much the best idea if you are able to set up the cleat so that it is laterally positioned in such a way as (for steady state efforts) you don't really
need stabilisers; hence my comment earlier about being very careful about this setting and even doing test rides on pedals which don't offer any support outside the cleat area.
If you have the support but don't really need it, such things as wear will both be reduced and of less significant effect.
A large fraction of SPD users would benefit from more space between the pedal centreline and the crank. I mentioned packing washers before but for larger changes (thus converting a standard SPD pedal to about the same spacing as the longest speedplay pedal spindle more or less) pedal extenders are a good idea. They cost about £15 and can be used with any 9/16" threaded spindle. Unfortunately the minimum length of the extender is about 14mm or something, so no good for small changes.
It is also worth just mentioning that when shimano were pushing two-bolt SPD for road racing (circa ~1993 ?) they produced two pedals Dura-Ace PD-7410 and Ultegra PD-6500 which are different from any SPD before or since; they were designed for cleats SM-SH70 and SM-SH71 (although they will work with SM-SH51 etc) and they have ~53mm offset of the pedal centreline from the spindle shoulder. This gives ~4mm narrower Q, if you have narrow enough feet to be able to use the reduced offset.
PD-6500 ultegra SPDPD-7410 complete with cleats2002 vintage shimano 'road' cleat application table, including SPD-R cleats (remember them?) but also showing SPD cleatsOne of the big differences in SM-SH70/71 is that the lateral adjustment available for the cleat is very much greater than with SM-SH51/56 cleats; however you can only take advantage of this (and fit the stabilisers of course) if you have non-recess type shoe sole.
It is also worth mentioning that in ~1997 shimano revised their SPD pedals; they retained the part numbers SM-SH51 and SM-SH56 for the standard SPD cleats but before and after this date the cleats themselves are different as are the pedals themselves. New cleats in older pedals means less float. Old style cleats in newer pedals means more float than normal. The revised claw design is the big difference between (say) PD-M323 and PD-M324.
cheers