Well without your thumb round the back you couldn't very well do sliding barre chords anyway!

With some guitars, and with some hands it's not possible to thumb the 6th string anyway. But it's worth being able to have it available. Without it there are some sounds you just cannot get, no matter how great your guitar technique is.

But equally if you can't do full barre chords you're limiting yourself too. Recently I've been working on strengthening my left hand, using a small foam rubber ball, and it's surprising the difference it's made - I can easily do things I'd struggled to do for years. And I've been able to dispense with putting a strategically placed sticking-plaster on the first joint of my index finger, which was my way of eliminating a buzz on the third string when I was playing barre C shapes.