It's a fair point that "circuit training" can be an idea that varies wildly depending on who's advocating it. I actually work at a health facility so it is possible I am applying my biases to what people mean by the term.
As an example of circuit training that is NOT the "usual bullshit housewives at your local health facility are doing," while I realize most people around here probably don't look so highly on HIT (disclaimer: I am not a HITer), Arthur Jones' original "circuits" were composed of exercises like barbell squats, weighted dips, and stiff-legged deadlifts. That is obviously another thing entirely from how a lot of personal trainers are going about the idea, or "feeling the burn" on a handful of shitty, selectorized machines.
Now, as to your hypothetical scenario about the bride-to-be losing 15 lbs, I think we are running into an unnecessary either/or scenario, "excluding the middle," as it were.
Meaning, just because a person might explode doing verbatim SS if (calories in) were too low or (calories out) were too high does not mean you can't still implement a heavy-for-her resistance training program as a component of her fat loss, and then include additional metcon work, interval training, or even *gasp* good old fashioned steady state cardio on top of this.
Bear in mind there is a tremendous psychological component to training a person, meaning amongst the various iterations of "what works," a person will have preferences that are only possible to discern by working with him or her. Any given person will also have a variety of contraindications that have to considered, too.
As above, I would still recommend a resistance training program that is a bit easier to recover from than SS as the backbone of her routine for reasons I've already outlined, assuming we have concluded that SS will be too hard to recover from.
Getting more specific, some compromise can probably be met via modifying volume (e.g. 2 sets of 5 instead of 3), modifying frequency (e.g. squat 1-2 times a week instead of 3 times a week), or even modifying intensity/load (e.g. push the rep range up slightly to something like 6-8, or 8-12 if need be). Note that, in terms of priorities, however, load on the bar is the thing I'd want to sacrifice last. What might such a routine look like (a bit arbitrary, JUST an example)?
A
-
Squats - 2 sets of 5
Bench - 2 sets of 5
Rows - 2 sets of 5
Weighted Situps - 1 set of ~10 or something
B
-
Squats - 2 sets of 5
Press - 2 sets of 5
Assisted Chins - 2 sets of 5
Deadlift - 1 set of 5
Something like a Monday - A, Thursday - B type schedule. A routine like this could probably be knocked out in a 30-45 minutes on each day including warmups/cooldowns, and we'd only be committing two days a week to the weights.
My point here, even if you dislike the above routine, is that there is clearly going to be some middle ground where, even if something like SS isn't doable, we can find some compromise of training variables that allows a person to still integrate a basic, strength training component to their fat loss efforts.
I think an important point that might be overlooked here is that a beginner is in the somewhat novel position of being able to simultaneously gain strength/muscle and lose fat. As such, foregoing a basic, strength training routine in favor of higher rep, "circuit" type affairs (where the goal is something other than gains in strength) does not seem the path of wisdom to me. That is, of course, just my opinion.
So, with this backbone of resistance training in place, where our subject is training with weights twice a week, I'd add a couple/few days of "energy system" work to the mix. Metcon work, HIIT, tempo training, steady state stuff - it depends on that person, their likes/dislikes, and their contraindications.
As an example, if our bride-to-be was a rather large bride-to-be, having her do any form of sprinting/jogging would be a very poor idea, considering the impact forces on her already-large body. I would much rather somebody like this do something more sparing to the joints, even if that meant getting on an elliptical 45 minutes a few times a week.
Some people find the variety of crossfit type stuff to be more enjoyable than "normal" cardio, and assuming we've determined it's safe for him or her, I would have no problem with our "energy system" work being metcon in nature. Depending on how heavy of loads are involved in this metcon work, we might be able to get by on an even more abbreviated resistance training routine. If our "metcon" is flat out crossfit, where we are already doing 5 x 5 or max effort work in heavy compound movements as a component of our training, a case could be made (particularly for beginners and the over-fat) that our metcon is, in effect, also our resistance training program.
As a last consideration, I'd offer that fat loss goals, in my experience, are largely met through diet modification. I wouldn't call training unimportant, but, as a bodybuilder I know used to say, "abs are made in the kitchen." All the metcon/"circuit" type training in the world is very, very easily undone with a donut here or there.