Pets called using the Call Beast ability do not subtract from the Beastmaster's experience points. (The Aug. 27, 2007 Version Update removed the Exp penalty from charmed pets as well.)
Jug pets are significantly weaker than wild monsters of the same level; they don't have a subjob, so their attributes are about 25% lower.
Jug pets are weak for solo play. They work best as an addition to a BST's damage output in party situations. In an XP party, a Beastmaster with the proper jug pets can be extremely powerful. Consider it like adding another member to your party of a level or two below which doesn't affect the XP of the party. Most jug pets will last their full timespan without ever taking damage (barring strong mobs with damaging AoE attacks). If a BST knows what prey your party will be fighting, they will most often be able to bring jug pets that correspond to the mob's weaknesses, at least at lower levels. HQ jug pets are expensive, but necessary at higher levels when other jugs are capped.
Jug pets are significantly stronger against easy prey and lower monsters, making them very effective for farming and soloing in non-xp conditions (where monsters are within a few levels of the Beastmaster)
Aug. 27, 2007 Version Update: Pets summoned using the Call Beast ability will now remain for a longer period of time; the duration varies depending on pet type.