Ideally nothing breaks and what gear one brings is entirely dependent upon the requirements of the assignment. However, in reality accidents might happen and the emphasis then shifts to risk assessment and damage mitigation instead. Whether the better plan is additional camera(s), or extra lens(es) more or less overlapping in range or functionality, or both; will vary and only the photographer can make the final decision in this regard. Perhaps this aspect is moot due to the lack of availability of additional gear anyway. A "minimum" kit comprising 2 cameras and at least 2 lenses will allow *some* residual functionality even were a camera or lens to malfunction.
In practice the concerns outlined above rarely or never are manifested and only when a crisis emerges, one faces the consequence of whether or not having considered alternatives prior to the shoot. Any 'pro' photographer ought to have contemplated such risks in advance. The is a vast difference between returning with (sub)optimal images, or none at all. A client is not interested in what went wrong - she or he wants results.
Totally agree with the above statement.
An amateur can, and in some case as I experienced, will have much better equipment then a pro (me in my case), and at times may be able to shoot (some) pictures that are 'better' then what the hired pro was able to shoot from the same subject/event.
But the difference indeed is that a pro will always/is always expected to come with successful/quality pictures, whereas an amateur can just play along, bang his chest if his pictures are 'as good' as the pro's, but does not have to take any responsibility if they're not (usually with the excuse 'but I'm not a pro' or 'but he has better equipment') and just fade away.
And yes, that translates in the pro always having to take extra equipment along, just in case of failure, or of something unexpected popping up for which he might just need that one special camera, lens, or whatever he has somewhere in the back of his closet.
When I go out for eg shooting sport (pretty rare nowdays fortunately) I have my 7 fps (that's about the fastest rate I use) D3 as a first pick, but also take along a D800 with grip, which in DX mode allows 6 fps)
For a shoot which requires high res files, I take along 2 D800's, and for a high ISO job my first choice is my DF, with a D800 (I know 'only' ISO 6400) as back up.
Similar with the lenses, all of which in the end results in having to 'schlepp along' a big bag of equipment, most of which often enough (fortunately) is left unpacked without any use at the end of the day. But if something does break down, big sigh of relief that you can simply reach in that bag, and continue as if 'nothing has happened'.
That's what clients are paying a pro for, and if something that nowadays is too easily forgotten when they hire a ' I started photography two years ago and have a DSLR and a big lens' low price 'photographer'
http://emgn.com/entertainment/amateur-photographer-turns-late-takes-selfies-ruins-couples-wedding-day/