Yes people do tune there guitars this way, and making a comment to say it's "wrong" would be arrogant. Who is he to say how to tune a guitar? Who made him God of the guitar to say how low someone can detune there guitar?
There are legitimate musical genres that rely on detuned guitars to drive their riffs. HEAVY METAL and several sub-genres like Black Metal and Death Metal where guys need a lower tuned guitar to get a deeper, darker tone is an example of this.
Then to say that your strings will be too loose to play is also ignorant. Yes, if your guitar is tuned to E Standard, you shouldn't detune to A D G C E A (also called A-Standard) because your strings will indeed be very loose and your guitar will not be in tune all the way up the neck, however, you can adjust the intonation and bridge to adjust for this on many guitars.
Has this guy never heard of a 7 or 8 string guitar? Those aren't tuned to E standard. A 7 string is tuned to B standard. What, is he going to say something like only Metalheads use such guitars and they don't know how to play the guitar if they are in this tuning? Well, here's some information for you, George Van Eps built a 7 string guitar in the 1930's. He didn't play metal, he played Jazz.
Yes, it is called A Standard, because you're keeping the same "patterns" as you suggest. When ever you have a standard tuning. Taking the low E string (or the B string on a 7 string) and dropping it 1 full tone (equal to 2 semi-tones) you will have a drop tuning. You can be in B standard on a 7 string and drop this 1 string and be in what is called Drop A tuning.
People shouldn't confuse this with an Open A tuning. If you tuned to an Open A, and wanted to tell people which tuning you were in, you would say, "I'm in OPEN A tuning". If you said A standard, you would be using A D G C E A.
And NO, putting a Capo on the 5th fret won't net the same results as tuning to A D G C E A. If you were indeed after a lower register on the guitar, you would need to adjust the tuning.