Diversity is the better way.
First of all, each of us developers (and users, too) has a different preference on the machine we want to use. Some want a small laptop to use on the road. Some want a powerful workstation. Some just use what they can afford because they don't have a big budget.
When I work on WebKit, I need a powerful machine, with many gigabytes of RAM and a lot of diskspace on an SSD. This is far from the typical configuration needed for a normal user. Also, the machines I use currently probably aren't being sold anymore, so even if you wanted to get the same machine as me, you couldn't.
If we agreed on a single machine, our software would get a lot less testing on any other. So we would end up with one well supported machine, and nothing working on anything else. This would further reduce our userbase because you could not try Haiku by just a download, you would need to buy a dedicated machine just for it. Who is going to spend hundreds of euros on a machine just to run an hobby operating system like Haiku? (ok,these Amiga enthusiasts may, but they already have at least 3 different OS to chose from!).
In my case, I just run Haiku on hardware I happen to own. Some machines I could chose (for example my desktop PC I built from parts), others people donated to me or I just got a very low price offer thanks to a group buy. There is no reason I would synchronize with all other devs - not to mention some of these machines may not be available in their part of the world.
So, a lot of added complexity and coordination, with the only goal of less support for different machines. Because of course when I get a new machine, the first thing I do is fix the drivers to get it running!
If you want a supported machine , just ask one dev (any of us) what they currently use. This is why we need an hardware compatibility database, so you can see the status of different machine and see if there is one that does everything you need supported.