(Whoops, turns out I accidentally wiped out a paragraph or three. Small keyboard... fat fingers... rickety train...
The gist of it was that an SBEC is used instead of the ESC's BEC, and not only if the BEC should fail. Hence, an SBEC doesn't add redundancy or "failover" capability.)
...be more reliable than the SBEC, using the latter will obviously decrease safety to some extent. The hard part is determining which is more reliable
An SBEC can have other advantages. Once you go beyond the ubiquitous 48" wingspan 3D airframe, you're talking about some serious servo torque and speed - much, much greater than warbirds and gliders of similar size. All that servo grunt comes at the expense of power consumption. A BEC (whether integrated or not) which is adequate for a 55" warbird may be totally overwhelmed by the servo loads of hard 3D on a 55" airframe. Past a certain threshold, BEC voltage sags to the point where the receiver effectively goes offline - a "brownout". Voltage will usually recover within milliseconds, but the receiver will typically take precious whole seconds to reboot, initialise, and arm, by which time the plane may be digging divots or in the reeds
Hence, the advantage of SBECs is that they can be (over)sized according to servo loads, which is especially important if using high voltage (HV) servos and their even higher power consumption. Their weight can also be placed in a versatile manner, to help achieve a particular CG.
All of that goodness is in doubt if the SBEC happens to be a dodgy low-end unit which isn't as reliable as the built-in BEC, whose dormant weight is meanwhile being uselessly carried in the ESC. The SBEC's extra wiring and complexity should be taken into account as well.
Another strategy is to forgo the SBEC and instead invest in an ESC with a high-powered BEC. Many of the ESCs which fit that description come from the heli world, because it takes prodigious servo power to move the swash plate around. Brands like the German YGE and Kontronik have an excellent and well-deserved reputation, but their price induces nosebleed (especially Kontronik!), and it may not make sense to use an ESC that costs more than the rest of the airframe put together. Hobbyking's YEP line are YGE knockoffs, right down to circa-2008 YGE firmware. Their components are somewhat inferior to YGE, but it's still a lot of ESC for the money, and arguably far better than anything else in the "budget" range. Most importantly, their 6A switching BECs will handle any load a 55" airframe can generate.
Another option is the Castle Creations Talon 90. It has a 9A continuous BEC, 20A burst, but it's probably overkill for this plane, unless you're planning on using a 4S or even 3S power system (not recommended), and hence high currents.
Lastly, HV servos can also be powered directly off a small 2S lipo. No electronics to fail, but you typically get 3 or 4 flights before the auxiliary lipo had to be charged, and forgetting to do so is not a good idea. There's also the weight to consider, and the inevitable reduction in servo performance as the lipo voltage drops off.
In the end, there's no one-size-fits-all answer. It depends on airframe and equipment specifics, and personal preference.
paaatience !!
paaaaaatience 
paaaaaaatieeence 
dooooohhhh !!! 
