As little as I care about the front pockets of trousers on a lounge suit, I care even less about the rear pocket(s). To me, I never use the back pocket on my suits (I only have one back pocket, on the right) for anything. Therefore, I don’t even know it is there.
First, although in this day and age we all take our suit jacket off at the office, when we are in public a gentleman will almost certainly always have his jacket on. As a result, nobody can see what your back trouser pockets look like (or if, indeed, you have any pocket at all).
When you are jacketless, however, like with any small details on your suits, you may have a preference. As I said, I have one rear pocket on my suit trousers — a welt pocket with a button. It’s a superfluous detail, but my first bespoke tailor in London suggested it and who was I to disagree?
You could, however, argue that because it serves no purpose (a bit like the lower button in a two-button single breasted suit), there’s no reason to have it. While I do feel strongly that a two-button single-breasted suit is definitely not for me, there was a point made by one of the Mogg chaps in favour of having a rear pocket (or pockets): without a pocket(s), your backside may look feminine. If that is a worry/concern, then definitely go for pocket/pockets. If not, consider going without what is essentially an adornment on the back of your trousers.
Beyond the simple welt pocket I’ve described above, there are several other pocket styles that you may want to consider if you’re that way inclined. One of our favourite films at Mogg Towers is the original, 1974 version of The Great Gatsby (with Robert Redford, Sam Waterston, Mia Farrow, Bruce Dern et al). In that film, for which Ralph Lauren did a lot of the men’s clothes, the gents had two rear pockets — one piped pocket, one with a flap — which is a very cool, 1920s look. Speaking of old school flourishes, these pocket details also feature in some vintage formalwear by Ralph Lauren.
Whatever you decide, though, let me reiterate that you should not use the back pockets on your trousers for anything. Don’t shove your wallet, or anything else, in your back pocket(s). Your trousers can look elegant with or without pockets, but you’ll look awful with the pockets bulging with stuff in them. BTWB/MM/YS