Everyone's needs and tastes are different, making it difficult to tell you simply what works for each one of us.
- I can tell you that in countless trips to Europe for business and pleasure, I have found that on 135 format 28mm simply is not 'wide enough' for me, and I found that if I travelled even with 24mm (as my ONLY FL), I was happy!
- The other thing I can tell you is that (based upon travel experiences accumulated over 25 years with film cameras and only ISO 400 as the fastest film, it was important to have fast max apertures, because in so many places you cannot use flash or set a tripod!
Digital makes things a bit easier in some ways, but worse in other ways
Where I might use f/2 WA fixed FL with ISO 400 film 25 years ago, today the zooms predominate with f/2.8, or f/4 owned by so many and most digital cameras easily go to ISO 1600 without too much objectionable 'grain' (which i think the average photographer is simply too sensitive to the issue...but I grew up with ISO 400 film grain!)...but f/2 ISO 400 is no different than f/4 ISO 1600! It isn't until you get a camera from the past 2-3 years or so, with their 5-digital and even 6-digital ISOs that relief from the need for a fast aperture lens is diminished signficantly.
So what I ended up carrying in my Olympus OM bag was a 24mm f/2 lens, for less frequent indoor need I had 20mm f/2, and then a 35-70mm f/3.6 zoom
Today, shooting APS-C body, I have an 11-16mm f/2.8, with 17-55mm f/2.8 or 28-75mm f/2.8, a 100mm f/2 (but which often stays at home), and a 1.4x teleconvertor
I used to feel the 'need' for 135mm f/2.8 very long ago in my film days, and I think a zoom is so often a usual first 'additional lens' for so many hobbyists. I now take my 70-200mm zoom on relatively FEW occasions and virtually never when flying is in the itinerary. In travel with the dSLR I usually find that the more compact 100mm f/2 + teleconvertor affords long FL but with a lot less conspicuousness ...I don't need the thieves patrolling the high tourist traffic areas to be targeting me because my long white lens makes me look like I have a sign "tourist...a mark for theft opportunity!" The 28-75 zoom with 1.4x convertor provides me with an AOV equivalent to 168mm on FF, and that is plenty fine for me for most needs.