This is the question asked in the OP, albeit different than the title.However, the OP SUBJECT asks a completely different question: “effects cable” and instrument cables ARE the same thing and ARE interchangeable.
Is there a difference between speaker cables and instrument cables?
I think this is somewhat overblown on the internet.And a mild comment to add to @mhammer ‘s: at low volumes/power levels and short runs, using an instrument cable between speaker and amp isn’t going to damage anything, but at higher levels, you can melt the instrument cable which can have a cascade effect of running an unloaded tube amp which can result in amp (OT) damage.
You are technically correct, but most people would not do a careful calculation and evaluation in this case (or any case). Your example is a pretty good typical case, but instrument cables come all the way down to 24 gauge and at multi-core, can be rated as low as 1A, which is about 8W in your example (or even just 4W for a classic Fender 4ohm load or, even worse, 2W for some bass setups). And many speakers are worse than 100dB efficiency, so maybe 8W or 4W (or just 2W for bass) isn't all THAT loud. So as you say, perhaps it's overblown for a typical case, but caution is in order when it's so easy to deal with it and if you're playing outside your basement (or even in your basement after having lost most of your hearing). We really don't want people blowing their cool amps.Example: a 22 gauge multi-stranded center conductor of a shielded cable (the limiting factor) is rated for around 2 amps. At 8 ohms, that is around 30 watts. And 30 watts can be kinda loud, with a speaker rated at 100 dB (1watt*1m). Using an instrument cable for a short period of time, or in a pinch/emergency or whatever, isn't the end of the world as some would have you believe.
So...Be extra cautious and safe and play outdoors ONLY on the coldest days of the winter. Correct?!P.S. Also, if playing outside in the weather we've been having, you gotta apply the 0.82 current carrying capacity reduction factor for 30C+. 24 gauge 8W suddenly becomes 5.4W and etc.