Going from Illum to Makeb would often leave players under leveled, and under geared. Makeb did have a difficulty spike as it was designed keeping in mind that players for months had been 50 and were geared up already. So if you go from Illum to Makeb you will see a big spike in combat difficulty.
Also Makeb content was really worth only about 2 to 3 levels at most worth of XP, you still needed to do other activities and content to earn XP to reach 55. The bolster and xp boost they added was essentially to make the transition easier since they are now adding in another expansion. Otherwise players would get stuck in the leveling process around lvl 48-50 and struggle through Makeb to still be short of XP needed to move onto the next expansion.