The 12-40 Pro was my very MFT first lens, I got it because I bought the E-M1 kit which had it bundled. Back when I was shooting Nikon APS-C, I never really cared about primes at all, that more or less changed when I got the Pen-F and started discovering the beauty and convenience of the small and lightweight f1.8 primes. So in this regard, for me it was the other way around: The 12-40 Pro actually got replaced from having a save place in my bags to more or less mostly sitting on the shelf.
Having said that, I lately more or less rediscovered the lens and, while size and weight might not really be too convenient, the flexibility and versatility is. I especially again noticed how impressive the minimum focusing distance is, you get extremely close to any subjects and that works pretty much throughout the whole focal length, which means you can get macro-like shots on the long end or out of focus environmental shots of certain objects on the wide end.
To sum it up: Shooting habits seem to change, especially if you are new in photography, at least that's my opinion. I'm very glad about my small prime lenses and I am also glad about my 12-40 Pro, I'm not planning to sell any of them. Each one has a place in my kit, depending on my mood or what I might plan to shoot, I pack the lenses I like. Sometimes it's just my lightweight prime holy-trinity, sometimes it's my 12-40 Pro with the 75 f1.8 and/or the 9-18 thrown in. While the flexibility of a zoom is always great, I also feel by "limiting" myself with a small set of primes I start to discover different kinds of perspectives, which I wouldn't really try by just using a zoom.