The optional rule for doubling XP for sidekicks is meant to help the sidekick catch up to the rest of the party. A 1st-level character sidekicked into a 14th-level group will get enough XP from a single balanced encounter to get to 2nd level; with double XP, it will put him almost at 3rd level. With XP doubling, a 10th-level character sidekicked into that same 14th-level group needs only 3 encounters to level up to 11th. Assuming that you are going to stop sidekicking when you hit 12th-level (since a 12th-level guy in a 14th-level group is "close enough" to survive and be useful), with XP doubling it takes only 16 encounters to reach 12th-level, and that's not including quest rewards and so forth.
We might actually need an optional rule to slow down advancement. Maybe the sidekicked character gains at most 1 level per session? That means that whenever he gets a new power/feat/etc., he has one whole game session in which to master it before having to select a new one. (XP would continue to accumulate, he just wouldn't level up more than once per session. Limiting advancement like that actually sounds like a fine house rule independent of sidekicking.)
-- 77IM
We might actually need an optional rule to slow down advancement. Maybe the sidekicked character gains at most 1 level per session? That means that whenever he gets a new power/feat/etc., he has one whole game session in which to master it before having to select a new one. (XP would continue to accumulate, he just wouldn't level up more than once per session. Limiting advancement like that actually sounds like a fine house rule independent of sidekicking.)
-- 77IM
