The whole emphasis around the Bretonnia battle plan should be to get up 'close and personal' as quickly as possible. You may be concerned about the enemy's long range artillery capabilities but using a valuable rare slot on a trebuchet to counter them
is not the answer.
You must not stand off with the enemy when fighting with Bretonnians. If you include a trebuchet, not only do you miss out on your Grail Knights option but you will need to leave a unit behind to protect it. Therefore when you look at the Trebuchet cost of 90 points you should really read 190 pts because that's the value in point it will tie up.
In larger 2000+ games you might want to rethink this rule of thumb, depending on the enemy race but not in 2000 pts and lower.