What can't you understand jamezman??? That every character in the game has an EXP limit???. Try to play a battle more than five times and if you gain levels in every battle then on the fifth battle your characters will gain only 1 EXP instead of 49 or 36 or 24 or 10 EXP or what else. Promotion doesn't change this limit. You will gain 1 EXP from an enemy in both options (promo & not-promo). The main levels (which you can easily reach) are 35-40. Now it's your choice when you will promote your characters. For example if you promote Synbios at level 20 you will be able to reach up to level 15-20 as Soldier(first promotion) and you will need more EXP to have a good condition when you will recieve the Innovator spirit in Scenario 3 (Apostle of light class). Or you can promote him at level 10 and finish the first scenario with a Soldier Synbios level 25-30 with far greater statistics (cause of the class bonus points). It's simple.
There is no advantage for your force when you get further than level 10 and continue the game without having a promotion first. Of course you can promote them whenever you want but the best way is to get a promo asap.
-lid230
Keep going my friend. Noon will became one of the best wizards in the game and sorry for spoiling out Hagane!