Just kind of evolved that way really.
The 5" special pre-dates the Wee & SD990. I had that for "off-road" navigation when green laning but also used it when adv touring (mix of off-road & road mapping).
Screen is OK but sometimes, if it's a very bright day, it can be harder to see. Can also be sensitive to positioning, can reflect your jacket or helmet back at you which makes it harder to see the map.
I have the Sat Nav mounted on the bars in a tough otterbox for water & bash protection.
Then I bought the Wee and made a GPS bar above the dash and fitted it there in a ebay zip case. It worked great but it was a bit of a faff putting it in & out of the case, and the battery has almost died now so you get very little usage time to route plan if it's not plugged in & powered.
So I then started looking for a replacement, I wanted one that didn't need a case to be waterproof. I looked at getting another ebay one, one that also comes with a powered holder, all for £80.... but at the last minute decided I really wanted the Montana as it has the best sun viewable screen of any outdoors Sat Nav I've seen, so bought a used one. I then bought the powered cradle and wired that to my Wee (as the Wee is my touring bike).
So at that point I have 2 GPS's and 2 bikes.
Then I bought my SD990... I tried the 5" special in the holder on the bars but it looked bulky & wrong. I haven't tried the Garmin on there as the powered cradle and clamp system are a bit pricey. Instead I found a low cost phone holder, my phone is waterproof and has a secondary magnetic charge connection so I decided to just use that. I don't often need Sat Nav when I'm out playing on the SD, plus having the phone mounted & charging is one less thing in my pockets, and is a Sat Nav system if I need it.
So for now I'm sorted. I might one day put my Montana on my trail bike but for now I think it was too expensive to expose to that environment, however my friends do and theirs and they've been fine.
When touring I have the Montana on my Wee above the dash and my phone clamped to the bars, so potentially 2 Sat Nav systems, which is handy as one can give you more of a route overview which aids in dealing with unplanned diversions.
There you go... I bet you wish you'd never asked now...