Think about "spoiling the ship for a halfpenny worth of tar" or "an ounce of prevention is worth a pound of cure". If you had an interference engine where skipping a $30 idler pulley and risking a $3,000 cylinder head replacement makes it a "no-brainer", you probably wouldn't think twice about it, especially since most of the costs associated with timing belt replacements are for the labor involved.
Compare internal idler pulleys in the timing system with external idler pulleys on external serpentine belts. Just because the bearings are "lubricated for life" does not mean that they do not wear out in time; they can become wobbly, they can become noisy and occasionally they can sieze up completely. Most idler pulleys exhibit symptoms of wear by 100,000 miles, give or take a few thousand, and most timing belt change intervals are set for 60,000 miles. At first change, without replacing the idler pulley(s) you are risking whether you can make it to 120,000 without breaking the timing belt due to excessive stress; risking further idler life beyond 120,000 miles borders on foolishness.
It's not really a question of "need" when it comes to replacing internal timing belt idler pulleys but rather a question of what is prudent. By the way, when talking about your recent premature failure, you do not state how many miles are on the engine or whether the failure was deemed to be because of a bad idler pulley; only that the job was not done properly the first time.