Using your thumb isn't "bad technique" per se.
It's either appropriate or it isn't, given the context in which you do it.
For example, if this chord is sandwiched between two others which require the first finger barre, maybe it would be better to leave the finger down through those chords.
Bad technique is what I did when I started playing years ago, saw Jimi Hendrix using his thumb, and decided I didn't need to learn how to barre properly!
music.stackexchange.com/users/12961/Bacs
Who has the power to decide?
Any technique that works for an individual cannot be a bad one.
Yes, purists may disagree, but it's not them playing, it's you!
As stated already, an extra digit is always an asset, particularly on extended guitar chords.
I'm only jealous, having small hands...
music.stackexchange.com/users/2808/Tim
The traditional arguments against wrapping the thumb around to fret the sixth string are that it slows the hand down when you need to change fingerings, and that it brings the other fingers closer to the strings so they are more likely to mute other strings unintentionally.
This matches my experience; but if you can play quickly and cleanly with your thumb wrapped around, those reasons needn't bother you.
I find that when I try to fret with my thumb, or even just have my palm on the back of the neck, my wrist and fingers are flexed more, and there's more tension in my hand.
Even when playing a barre, my hand feels much more relaxed if I have only my thumb on the back of the neck.
This make it easier to play for a long time without getting sore.
music.stackexchange.com/users/32697/JohnTate
It is unorthodox, but it's not bad per se.
I'd definitely recommend learning to barré properly/use your first finger too, and then use whichever is appropriate based on the specific chord and its context.
A classical guitarist (or at least, a classically-trained guitarist) wouldn't really use their thumb like that at all, but plenty of top guitarists still do:
Metheny, (Jeff) Beck, Emmanuel, Hendrix, SRV, Bonamassa, etc... Loads.
I find it does produce a slightly different sound - somewhat warmer and fluffier than using your first finger.
This is because your thumb is going to be quite flat against the neck, whereas your first finger would be more pointed (i.e. you would be using your fingertip more).
There are no downsides to this technique if applied appropriately.
However, if used as an excuse to not learn other fingerings, you might find it limiting when playing chords that demand a full barré or similar.
So yes, it's fine, but learn to play it both ways and then use whichever you feel.
music.stackexchange.com/users/32699/Bernard Woolley
I wouldn't say this is a bad technique.
There are many guitarists that use this one, for the very same reason you mentioned.
You have one extra finger, thus you can play one more note.
Also, when I play a chord like this, it is more relaxing for my hand.
music.stackexchange.com/users/7356/Shevliaskovic
Not at all, it's a great technique!
There are chords that you can only play by using your thumb, e.g. the following voicing of A13(b9)
(from low E to high e):
5 X 5 6 7 6 where you play the E string with your thumb.
If you used your first finger to play a barre chord it would be very difficult to mute the A string.
music.stackexchange.com/users/9724/Matt L.
Some of the tricks used in country music where the guitar was for show more than anything else, the would wrap their thumb around the three base strings and the index fingers around the 123 in same fret to form a chord.
Then moving it up and down the neck the singer could play along with the band.
Some of those country singers playing was worse than their singing.
I use my thumb watch some of Johny Cash videos
he uses the thumb
music.stackexchange.com/users/23274/user23274
Try playing F#dim with your thumb on the bass string to get the correct inversion, and then it’s obvious that this one’s a no-brainer.
Yes, it does slow you down so it should be avoided, but in certain circumstances it really opens up the instrument’s full potential by getting the correct inversion and using all 6 strings.
music.stackexchange.com/users/61318/Andie W
I play steel string acoustic guitar
and I find that in many instances using the thumb to barre is actually more ergonomic than using the index finger.
Typical use case for using the thumb is when I want to play or mute the low EA strings while simultaneously playing or chording on the GBE strings on a steel string acoustic guitar.
In this case if I barre with the thumb, then I can keep my wrist straight much easier and with less finger pressure than if I use my index finger.
YMMV, of course.
music.stackexchange.com/users/10635/pro
When I play barre chords using jazz voicings such as in the following chord chart:   I often wrap my thumb around the neck to fret the E string. I prefer this, rather than using my first finger, as it makes it easier to play more complex chords since it frees up an extra finger. I want to know whether this is considered 'bad technique' and whether this has any negative long term effects on playing.
Is using your thumb for jazz barre chords 'bad technique'?
music.stackexchange.com
<guitar><chords><fingering>
music.stackexchange.com/questions/36338
music.stackexchange.com/users/8482/Tim Hargreaves
Using your thumb on guitar to get the chords is unorthodox, but it's not  bad  per se. In fact, this technique is used by many guitarists and there are no disadvantages to it.  It's generally agreed that using your thumb to fret to bottom string is unorthodox but certainly not 'bad technique'. 
It's generally agreed that using your thumb to fret to bottom string is unorthodox but certainly not 'bad technique'. 