Well... For some things, yes it's a bit annoying. But for others...

Say you're a lvl50 whm, standing in front of a Fate where you'll be synched down to lvl40. You cast Protect and Stoneskin, and synch down. Since you have both those spells at lvl40 (including the upgraded Stoneskin), it makes sense you get to keep them, yes?

Except... WHM Stoneskin gives you 18% of HP at casting time as a shield. So if you were at 4600hp before synching, and 1500 after synching (examples only), that would give you an unfair advantage against the unsynched people with Stoneskin on, since you'd have a shield of 828 instead of 270.

And while yes, they could probably go through all skills and check "ok, this skill doesn't change after this level, so as long as you don't synch below it, you get to keep the buff". But really, that would take an inordinate amount of time and programming, and therefore it's much more expedient to just make a blank slate after synching.


Edit: As for losing buffs when not being synched, that's a different thing and is most likely a bug that you should report :3