In mine humble opinion I think a 18-105mm is excessive, you are trading convenience for image quality. I have only use a 18-50mm f2.8 or 17-55 f2-8 for all mine travelling photography needs depending on which camera I bring along. What is important is that you get a tip top quality first shoot and after which you can crop the image for better composition. F2.8 is handy too for low light situation or indoor shoots. I have never desire for a longer zoom instead at times feel the need for a wider lens. Just sharing mine personal opinion.
It really depends how you use the zoom lens. If the intention is for pure laziness, then these people are missing the point. I use an 18-200VR for most of my travel shots, the other lens is the 35/1.8DX. The versatility allows for shooting at different perspectives and allow me to choose how much background I want in my picture. The thing about travel photography is that sometimes, you don't know what you're going to get and versatility is more important unless you are prepared to lug half your dry cabinet around. ;p
The other reason for using the 18-200VR is because of the D90. The 11x zoom is exceptionally convenient for that occasional video.
You are right, shooting a bit more and cropping later is good practice but you can only crop so much, so I try not to crop as much as possible. I crop and re-frame a lot during film days, but somehow after doing that for many years, I tend to automatically compose the frame during shooting until I find that I rarely need to crop my shots in post anymore. Also, shooting landscape, you can take your time to compose, unless sunrise/sunset is part of your shot and you only have a few minutes to capture that moment.
My experience with 3rd party lenses (with the exception of ZF), however, is that the optics are not as sharp fully open as Nikkors, which entirely negates the purpose of having it 1-2 stops faster while having a shorter zoom ratio. Yes, having f/2.8 is useful for low light but it buys at most a stop or 2 (depending on the focal length compared to a f/3.5-4.5 or f/3.5-5.6 zoom). In this aspect, a fast prime eg, 35/1.8 buys 2-3.5 stops which is more worthy. At f/1.8 background is mostly thrown out of focus, so the possibility to move closer or further for composition is greater compared to a f/2.8 zoom which may not throw the background out of focus enough and can be distracting.
IMO, f/2.8 zooms, however, are great for event shots where you get a balance of convenience and a faster f/stop. When I shoot weddings or events, 17-55/2.8 is definitely one of the lenses I bring as part of my arsenal.