pro tools is so great because of how powerful it's sortware engine is....LE versions are great for personal or home use, but once you start getting into big studios, the HD systems have changed the way recording is done completely....
first of all, on HD systems, you never need to worry about your computer slowing down or freezing up because ALL of the ProTools DSP operations are done by the Core Card that comes with an HD system, not your computers processor....if the HD card runs out of DSP, then you can add up to 7 Accel, or Process cards. Not only that, you can have a maximum of 96 audio inputs (HD 3 Accel) in ProTools. When was the last time you we reable to cimultaneously record 96 mic signals on Cubase?? For basic things that most of us are doing, that would never be necessary, but in major studios, when you got clients mixing huge amounts of tracks or Audio-Post sessions with 5.1 surround, nothing can beat it....Not only that, but in an HD setup, you can use hardware inserts, so if you have a nice vintage tube compressor u wanna use, you can do it....Also, all the major companies who develope plugins basically all design them around the TDM, HTDM or RTAS formats that ProTools uses. Lots of companies also make VST plugins which arent supported directly by pro tools, but other software developers have created "wrappers" that convert any VST plugin into an RTAS format, so basically, ProTools will run just about any audio plug in format you can find
Lots of other software companies have programs that can do many of the things ProTools can do, but I can guarantee this, no program will do it as fast, or as efficiently.
But the bottom line is, you gotta work with what's comfortable....If you put your sessions together in Cubase, any studio will still be able to bounce down your tracks, and import to ProTools, so it's not that big a deal, but when working in a studio, it saves engineers lots of time and money by using PT