Would you bother with the 12mm F2, 17mm f1.8 or 45mm f1.8 if you had the 12-40 f2.8 Pro?

When I switched over to m43, I purchased the 12, PL25, OL45, and happily used them. The PL25 was and is my favorite lens IQ-wise. When the 75mm came out, I bought it as well - great great lens. I later bought the 12-40mm used off ebay for a good price. It's now used most of the time for travel. For non-travel, I use the primes.

My 12-40 is as sharp as any prime up to 35mm. My copy is a bit disappointing at 40mm, maybe something amiss with my used copy, don't know. It's near macro capability is very nice to have as well. It's a very versatile lens, and it's nice not to worry so much in a light rain. It is heavy for m43, but compared to just about every other f2.8 standard zoom for other formats, it's quite small. If weight is an issue maybe look at the 12-35 for near equal IQ, but give up some reach and the near-macro capability.

Now even though the 12-40 is my main travel lens, I do admit that something in me misses the primes. I don't know exactly what it is, maybe I'm just trying to convince myself of something that doesn't really exist, but I find I like the rendering of the primes just a little more. And even though the size/weight is small for an f2.8 lens, I miss the form factor of the small prime on a small camera, which is why I got into m43 in the first place.

Now if I"m being honest, my 9-18mm at the same f-stop is dang near as sharp as the 12-40 - especially at 9 and 12mm. Most any lens is pretty good a f5.6. My next trip, I may decide to leave the 12-40 at home and travel with the 9-18 and a prime or two.

Don't know if I answered your question, but in the end only you can decide if the 12-40 will work for you. In short, it's as sharp as most any prime (mine up to 35mm most everyone else says 40 is fine as well), so yes, it can very well serve the purpose as a do it all standard lens.