Of course thinking you are hearing exactly what a different speaker sounds like, over youtube, through your own speakers, is going to be foolhardy for obvious reasons already pointed out.
On the other hand we shouldn't take skepticism too far. It may be essentially incredibly improbable to hear "through" your own speakers the exact sound of other speakers recorded in another room, but that doesn't mean that no relevant, informative sonic information about the character of a speaker can't be captured in this way. That would of course make zero sense - one may as well imply microphones (and playback systems) can't capture salient characteristics of the sounds of anything...from the differences in human voices to a horn honk.
If you play two different speakers in the same room, and those speakers have different sonic characteristics, that will in all likelihood be captured by a decent microphone, and be relayed through a decent (the more neutral the better) sound system. I think this is best suited for indicating some relative differences between speakers, moreso than a single speaker's character. No substitute for actual in person listening by a long shot, but on the other hand, not in principle utterly bereft of information. I've certainly heard all sorts of relative differences in youtube speaker comparisons. Some of which match pretty closely to my having heard those same speakers in person.
I actually enjoy some of the youtube speaker videos, mostly as sheer entertainment.
In fact just a couple nights ago I was watching some youtube Axpona tour videos (and other audio shows) in my home theater. The combination of the big super clear images on my projection screen had a "stairing in to that room" feeling and the sound quality through my home theater system was suprisingly clear, rich and "realistic" in sonic size. The sound shape shifted in character with each different speaker demo, really changing the sense of acoustics and sonic character with each room. The ILLUSION was fairly uncanny of actually visiting those rooms. As I say, fun for entertainment purposes.