The problem is that when BST go xp, for a lot of levels, you roam in an area radius larger than the pop area, or you have to drag a pet from pretty far away to your xp spot. It's rarely an option to stay near where the mob pop is.
Best pet to use is EM level pet since they do significantly better damage than DC pet and won't get destroyed by T mobs. So typically, you throw the EM pet at the mob, before pet dies, you tell it to leave and throw something else at the mob (most of time probably a DC mob unless you are really lucky and have a lot of EM pet at your disposal. After you kill the mob, your EM pet should be fully recovered and now you can use it on next mob immediately for xp chain.
After the change, not only you may have to travel far after each kill to find your EM pet, jugs don't cut it because the pet from jug typically are around DC level, pet food isn't sufficient to keep them alive for long, and there's cool down time to use jug and to use pet food, so the only thing BSTs are lucky to make xp chain on are EM or low T mobs. Imagine trying to get even just 20k xp for a level without being able to chain T mobs and having frequent downtime... That's what BSTs are complaining about.
Len: I doubt that will work. A BST + SMN party could kind of work with Diabolos' Nightmare ability (assuming it won't get resisted or something). But meh if this is the reason SE nerfed BST this way, someone needs to dump the designer in a pool of taru poop.