Ohh I don't know, I mean my Ubuntu Servers only serve 100's if not 1000's of clients daily. The websites that depend on my servers would argue its professionalism.
And for professional use, anything I need to do on windows, I can do on Ubuntu. While MS Office is prettier, LibreOffice does everything one needs... (minus Access, nothing beats MS Access for easy databasing)
Gimp does fine for photo editing, and nothing I have found can mass change any picture format to jpg as quickly as Linux... Which I use as part of my server install.
I mean, I don't know what professional apps you are talking about. But I imagine by professional, you mean, the apps you use on windows/mac/whatever os, and if it doesn't have that its not "Professional"