Actually romper its more like tons of camps you dont have to travel to for 30minutes+, weaker monsters that give more exp than theyre supposed to, extra refresh,faster pop times,double food duration ect..
theres a long ass list of reasons to to use the expansion to exp over the old areas unless literally EVERY other spot in the expansion is past its max capacity or you make a pt specifically designed for a non toa area(lufaise).
Once a person gets past lv 55 I'd suggest getting the expansion because I'd pick expansion areas to pt over non expansion and kick out the person without em unless they were my only choice.