I look fucking good in a suit, so dear god yes do I wear them. Ironically, my work attire is jeans and a wacky t-shirt (programmer), so I pretty much only suit up for when hitting the town.
And yes, pockets are meant to be opened, they wouldn’t be sewn up so shoddily if they weren’t. On the other hand, don’t place anything other than a handkerchief in the breast pocket,
I’ve never owned a new, tailored suit - all the suit jackets I own are used ones I bought at thrift stores.
That being said, i’ve always been of the understanding that the pockets on suit jackets are strictly intended for show rather than for use - actually carrying things is what your overcoat and pants pockets are fore.
Pro tip: in the circles where people wear suits as a matter of course, for work and for play, it is gauche to treat them as anything other than informal wear (because that’s what they are). You open the pockets, and use them when you want to. No one who matters is going to notice or care (or even understand) that that ‘spoils’ the jacket.
Thinking about it, if you want to keep your keys in the pocket of your morning coat or evening jacket no one is going to bat an eye at that either. Just open the pockets and relax about it.
I use the pockets. I have a couple of nice suits that have convenient sub pockets within the pockets, they wouldn’t be there if you weren’t to use them. Sandwich has it right, the trick to wearing a suit is just to wear it and not be the guy who walks around worrying about being in a suit, that guy always stands out.
I wear a suit or sport jacket a couple times a month, and regularly keep my keys in a side pocket and my phone in the inside breast pocket. I’ll also put other things in the pockets - mostly things that my kids or wife give me to hold for them.
I always cut them if I notice and get around to doing it – I have jackets that I’ve had for years with pockets still sewn up, but at any time if I’m near scissors or if the thread is starting to come loose I open it up.
On that second point…some here have been saying that the sewn pockets are intended to stay that way and maintain the line of the jacket or whatever. But IME the sewing is often loose and thin enough thread that it will unthread itself over time. And if anything, the more expensive jackets have the easiest pockets to open.