Resubmitting elsewhere without any key change when a paper is rejected While not necessarily being an expert, I have extensive experience in biomedical scholarly publishing as author (>500 scholarly papers published) and reviewer (>1000 manuscripts reviewed).

Most of my manuscripts have been rejected several times before eventually getting accepted and published, as this is quite typical of cardiovascular research. Indeed, I mostly dedicate myself to incremental research, such as meta-analyses and observational clinical studies, and often submit first to a top tier journal (e.g. with impact factor above 10), then trying to lower impact journals.

I now tend to favor not changing substantially my manuscripts after a rejection, even if accompanied by peer reviewers comments, as I feel that in most cases this has to do with a priority judgement rather than with the work strengths and weaknesses. Of course, I address major mistakes or issues, but I typically avoid adding ancillary but time consuming analyses in such cases. 

Is this acceptable and efficient, or actually unethical and disrespectful of the peer review process?
If the reviewers pointed out inaccurate citations, false or missing attributions, or statements in the paper that have been pointed out to be false or seriously faulty (and that you agree are false or seriously faulty), then it seems to me that you are ethically obligated to respond to these before resubmitting.

If the reviewers point out what they claim are deficiencies in your paper but these are subjective and/or you disagree -- e.g. poor writing or organization, insufficient motivation -- then it is up to you to decide whether to address these before you resubmit.  If you take them seriously enough, it is probably more efficient to do so, since the next reviewers could have the same issues.  In fact the next reviewers could be the same as the present reviewers, and if their serious critiques go unanswered, they are not likely to look more favorably on the new submission than the old one.

However, if the principal (or sole) cause of rejection was simply that the paper was not a good fit for the journal -- either by a mismatch of subject areas or simply by not being strong enough -- then it's plausible that this can be remedied by resubmitting immediately to a different journal.

When it comes to my own journal submissions, all of my rejections were said to be because the paper was not strong enough for the journal it was submitted to -- I imagine that the majority of rejections for sufficiently prestigious journals are for this reason -- sometimes accompanied by other commentary by the referees.  Probably more than half the time I have resubmitted without revision to a different journal.  This has worked relatively well for me: the papers usually get published after one or two more tries, and I have received absolutely no negative feedback about this.  In fact, most of the time the rejection letters I get explicitly mention resubmitting to another journal.  Sometimes they mention taking the referees' comments into account first.  When they don't, I read this to be a directive from the editor to resubmit without changes.