Pros and Cons, but Doable 20 Years Ago
There are good points made for and against, but after switching to the Linux desktop when SuSE still sold boxed-sets and switching the backend for 4 other law offices to Linux while retaining a mixed environment for secretarial machines on Windows it was all doable 20 years ago. At that point there were minor limitations in OpenOffice (and remain in LibreOffice) for legal briefs, Word provides the ability to create a Table-of-Authorities, you have to hack it in Libre. Second limitation was accounting software. (for small business that's still true, though Wine works fine there). Lastly, an exchange backend, that issue has gone away with groupware packages like eGroupware, etc..
The big Con point is the desktop itself and the knee-jerk changes to KDE or slow death of Gnome/Gtk from a thousand self-inflicted cuts. When something like "we need to port to the next Qt version for KDE-next, or we need to change Gtk/Gnome and remove/deprecate features of the toolkit -- there is no way business can handle the retraining/redevelopment involved. The fallacy of the desktop projected being "community oriented" and responsive to "community needs" evaporated with KDE4 and Gnome3. The emperor truly had no clothes and from a desktop direction standpoint the less than handful of devs guiding the project were going to do what they wanted, the community be damned. It's that uncertainty business cannot stomach.
The Pros, Samba does ACL, domain controllers and single-sign on better than windows. Linux can operate as controller or member in mixed backends just fine, that's a non issue. For small business, Samba in a stand-alone mode still provides an excellent file server without the DC while still providing fine-grained access control over any of the needed shared resources between groups of people. CUPS, handles all printing backend need from copiers, scanners, networked or shared printers and does it well. Groupware mean appointments entered on my calendar at the office with it reminders pop up on my iphone and new appointments and contacts entered on my iphone are saved to the database back at the office. MariaDB or Postgres handle the database needs for a small office, just as they do for airline reservation systems. There simply isn't any downside to a Linux back-office verses a MS one -- you just need someone who knows how to keep it going (and there is little to do there after the initial setup other than handle config changes on major version updates.
Not so much in demand now, but fax was another area where Linux shined. Hylafax with the Avantfax front end written by David Mimms (over at iFax) was a wonderful to with notification via e-mail and a link to the .pdf of the fax that had arrived instantly available.
There was a post above about a committed move to the Linux desktop on to revert back when a real-time inventory system didn't play nice with the new desktops. (though I'm sure behind the scene that had nothing to do with a technical problem that couldn't be fixed, but it exemplifies the lack of talent needed when a move is planned so you don't end up keeping inventory on paper to make sure the computer is right - for a problem cause by the "guru" who did the setup just not knowing how to get it configured properly.
Larger companies can partner with and contribute to open-source projects when they need features added. This benefits both the company and the project and them community at large if the added feature is relevant to more than just that company. Many open-source project will welcome additional help and contributions of funds or manhours.
Another big Pro is the cost for a complete Linux backend and Desktop setup is far less that the deal you will make with the Windows side. This is more attractive to small business running on shoestring budgets. You can have all the means of creation and integration on par with the top 500 companies of the land, you just need someone dedicated to knowing the software and systems well enough that when any of the Cons appear, they are not show-stoppers, and simple are worked and fixed just like One-Drive going down would require the same thing.
After 20 years, I have no regrets from embracing the Linux desktop, but still have quickbooks in Wine, so not 100% off windows, and over that time I've never had one software compliance visit from Big-Brother. There is another con in there somewhere.