<p>Currently, I'm using a Creality printer to print PLA (that's what I have on hand) but I'm definitely interested in working with other materials that require higher temperatures (both much higher, and just enough higher that the stock hotend is very marginal) in the future. </p>

<p>I understand that all-metal hotends are less forgiving and that they particularly are not the best for printing PLA, and shouldn't be assumed to be an upgrade when only printing PLA. </p>

<p>What I don't understand is, <em>how bad are they?</em> Are they so bad that I should plan on changing back to a PTFE hotend whenever I print PLA or ABS? Or are they suitable for use on a printer that is sometimes used for printing PLA and ABS and sometimes printing high-temp filaments?</p>

Are they so bad I should plan on changing back to a PTFE hot end?
No, all metal hot end are not that bad, and may even be beneficial when printing at higher temperatures. You mentioned that you want to print ABS and other such materials. At temperatures this high, my understanding is that the PTFE tube in the hot end may melt, or at least become so damaged that the hot end is blocked, leading to needing to replace the tube. My first 3D printer used a PTFE ho tend, which due to the printing temperatures I was using, and lack of knowledge of the many types of hot end, the tube got damaged after about 2/5 hours, leading to me needing to replace the tube. I eventually sent the printer back, got my money back and got a printer with an all metal hot end. That has never failed me in hundreds of hours of printing.
Are not the best for printing PLA
I have never had a problem printing PLA with an all metal hot end, however if you are printing just PLA, a PTFE hot end would be just fine, although not as versatile if you wanted to try different materials in the future.
In Summary:
Provided you print fast enough (I regularly print at 60 mm/s) to ensure that no filament cools down in the hot end you should be fine.