...I think the bottom line is use the best antenna you have available to you that is reasonable to use for the situation.
And best antenna
, of course, is a relative term.
For some it may mean the highest gain to a particular area. For others the "best"
antenna might be the one with the flattest SWR curve, or that fits in their
backpack, or works with available supports, or allows for multiband operation
without manual readjustment, or doesn't require a tuner, or fits in their shirt
pocket, requires minimal setup effort and/or thought, something they can
buy ready-made from a catalog, or perhaps the one that best fits their prior
assumptions of what a portable antenna looks like.
Often QRP operators will want one with minimum losses because they don't have
a lot of extra power to waste, but this isn't always the case. (I remember an
article on using a T2FD because the low SWR reduced feedline losses, conveniently
ignoring the fact that the inherent 4 to 6dB or more loss in the antenna itself was
much higher than the loss in any reasonable length of coax feeding it.)
Sure, efficiency and (to some extent) gain are important, but the other factors
may be just as important to some operators, or more so. Hence the wide array
of antennas that get used in the field (and various commercial offerings.)
We had a portable antenna workshop a few years ago, and I may plan to hold
another one again. We practiced skills for putting up antennas in the field
(such as tying knots, throwing ropes over tree branches, putting up portable
masts, etc.) and actually tried them out. That's a great opportunity to compare
performance and other characteristics of different antenna options, as well as
broadening one's pictures of what is practical in the field.