If you've not tried them, you wont realise that most of what is written above is nonsense
being able to use your legs on the upstroke of the pedals makes life a lot easier up hill. In the event you end up in trouble you tend to end up unclipped anyway. Walking is very shoe dependent, but there are plenty of shoes that are just as easy and comfortable (and of a similar style) to skate shoes. So much so that I often just wear my SPD shoes to walk around the shops or go to the pub...with no bike involved.
Cost, unless you ride cheap resin flats like those on super market bikes, you are probably already spending as much or more than a set of SPD's can cost. The shoes are an additional expense, I shall grant you that.
I won't lie to you, whilst getting used to them you will have the inevitable SPD related injury. Most often purely an injury to pride that occurs once you start to get confident in unclipping, pull up to a set of lights or a slow stop, then forget to unclip and end up in a heap on the floor looking a tit. But this phase tends to be short lived, nothing teaches better SPD use than embarrassment.
haha, that last part has actually happened to me, back in 98, I was accustomed to clipping in and out with ease, and never get it much thought, though I did have multiple bikes, and one bike had platforms. So, riding back from a trail, I was drinking with one hand, braking with the other, coming to stop light, in front of bus stop, and momentarily forgot I was clipped in. Came to a complete stop; couldn't grab the bar with both hands to balance myself, and slowly fell over sideways, in front of a line of people waiting for the bus.
I didn't wait for the light to change to green.
clipless is the way to go for any type of performance riding; there's no debate in terms of efficiency. If going with platforms, I know a lot of people look down on using them without clips/straps, but I've always found it safer to go without clips and straps, and have little problem keeping my feet on the pedals when jumping obstacles, bunny hopping, or maneuvering, so long as it is not slick, and if I have to bail- piece of cake.
Bailing with platforms/clips/straps; not cake, and not fun.
So it's clipless or platforms, but rarely platforms with clips/straps, though I risk incurring the wrath of some VRC folks.
As for clipless, my weapons of choice are Time ATACs. Can't stand SPD.