PWE want you, the level 50 player, continuing to open your wallet ... IF XP was easy to get, you won't ...
So they make XP harder to get so you have to log in regularly (keeps the number of players high as well) to do DoFF missions, but DoFF missions can chew through Dil and EC very quickly as well if your not watching. So you spend money on Zen --> Dil.
They also want you buying XP Boosters fro the C-Store.
If your doff missions are eating up your dil and EC, then you're doing something very wrong. If anything, doff missions generate dil and EC, especially for the KDF. And they generate huge amounts of EXP if you're diligent about daily doffing. I have several toons with 2mil+ EXP, gained just from doffing. No need for xp boosters or anything else from the c-store.
Pick up the zonewide missions from defera, especially the crafting ones. You're going to be doing the hard missions there to start with anyway, as its a very high reward/min of fleet marks relative to its entry level of skill requirement. Swapping zones will sometimes have a different set of zonewides. The goal is to be doing as many of them as possible simultaneously so that the crafting mats rewards from one fuel the completion of the others.
Doing a short, combat-heavy foundry mission once a day will also get you a reasonable bit of xp/min for a new-to-50 toon.