Is there any reason why I shouldn't use root as the default account that I log into?
Yes, sir, a major reason : we have a set of protections in Unix like systems, allowing us to live a quiet computing life avoiding major hassles, viruses, malware of all sorts, and several sets of right and permissions, as well as people who have invented policykit and polkit to allow us to refine even more these permissions.
If you login to X as the root user, you will have to use only the 2 or 3 tools that are meant to be used as root and nothing more. Even better if you never login as root into the graphical session (X session also named).
Each time you do so, you put your system at stake. Each time you will launch as root an application which has not been build for this purpose you will add to the danger for the system with possible default security (leaks, holes, many types of possible weaknesses). When such a program is started by the normal user in a normal user account there is no problem, because the permissions in that environment are limited. If you open such a program in an administrator environment, an attack can succeed, and a person or a bot attacking your system can gain root priviledges on your machine and do what they want with it.
Being user is the way to do : for root priviledges, you have the PCLinuxOS Control Center, the Synaptic Package Manager, for any item you don't find there ask the way to do this and that on the present forum, and follow the advice given by the people with experience.
GNU/Linux is a safe OS, under the condition the users respect just a few simple rules.