Macs are *nix. Going from macs, to SGI to linux to all sorts of *nix systems is just as simple as starting from linux.
GNU runs just fine on macs, thus so does fortran. I use it all the time. I specifically found it easier to write portable simd and gpu code in fortran on mac. I could do it on linux boxes but it ported poorly because everyones compile environment was different.
yes there are loads and loads of linux web site, that will offer you solutions that don't work on you configuration or maybe one of them will. But you don't know that till you try and find the problem has morphed requiring debugging all over again. Likewise when you come back to code years later and find it just won't build anymore on your new computer or the new distro you migrated to. I've wasted tonnes of time on that road. It's why I use macs whenever I can.
I've been very very productive on Linux too, but that was when I was in large groups that enforced a common setup and had a sys admin to handle all the networking and commonality of configurations. In such a situation, yeah you can get answers quickly from your neighbor for your linux mysteries. Outside that I've always found it slows me down and I spend too much time on the getting-it-to-work, and keeping it patched and not enough on the physics. If all you are doing is working in a company where you are writing c code and eating your own dog food (libraries) then perhaps that's fine. But if you are reading research papers and want to try out all sorts of different home built systems it gets hard to work on your own, this is where linux's diversity and obscurity kills it. There's problems with macs too I fully admit. That's where virtual box saves your ass. Sure you can fire up virtual box on linux too, but the one OS you can't get on the linux VM is the mac and all it's application goodness , easy updates and hardware compatibility.
As far as the tangential educational benefits from ones day trips into linux comment sites trying to figure something out, well macs are *nix too. So to the extent that things behave the same the linux sites are useful for mac-heads as well. And I suppose you know that mac osx compilers do cover multiple architectures including ARM.
Whether mathematic matters to you or not depends on what line of physics you are in. for mathematically oriented genres sometimes its a lingua franca. Others use matlab (especially in things like remote sensing or accoustics where coupling to hardware and rapid interface building is useful). Some just grind numbers on clusters and write their own codes for that. Just depends. Personally I don't use it anymore.
So yes, seriously!
I've never found it useful in the slightest to run the Linux just because the big cluster I use runs linux. The networking environment on the cluster is always so distinct that its as different as the Mac anyhow. It's easier just to Xwindows into the cluster or one of the develpment nodes set up like the cluster and just use that.