Well no-one is left off, everyone has the option to delete all their characters except 1 and take a lvl 30.
It is just that I feel I learned a lot of the tactical and positional awareness for this game from being low level, having to carefully pick my targets, run from people who looked too strong, stick with teammates who had better gear...etc...
Now that isn't to say people who use the lvl 30 to find out a build they want first won't learn those same skills, but they may get much more frustrated dropping down to low levels after only having played lvl 30 before.