I'm continually learning and to help me learn I will go on training courses - hopefully with the result of some lovely shots to show. I'm not one for group shoots, but do we occasionally participate in some of the EPS studio evenings.
For both of these, there is some part of the work for which I can't take the credit. The shot is mine and the edit is mine, but likely the lighting setup won't be mine and will be shared with other participants. While I will display work from these events, I won't be obscure about the source of the shots (and I'm certainly not going to enter them in a competition or add them to something I call a portfolio gallery).
I don't have any philosophical objections to using models in a portraiture (or even a boudoir) portfolio - those shots should be representative of the photographer's best work of in that style (and I don't see that as a bad thing).
Wedding photography portfolios are another matter - if the shots weren't taken in the context of a real wedding then they aren't really representative of their work as wedding photographer. That's important since a wedding only offers a single chance to get the shots and while a talented amateur might take some great wedding photos, you need to be able to rely on a professional to deliver on the day - regardless of the weather and all the unforeseen happenings.
That's just me though - what you see is what I take.