But Catapults and Trebuchets in Knight/Musketmen era? Not that much.'
Everything is situational. If there are knights that will flank my cats/trebs then out they go without a thought, cuirassier/cavalry against cannons...same result...but if the stack does not have flanking units...
Perhaps its just the way I play and defend but I often have fairly weak but numerous defenders in cities with a number of strong defender stacks ready to move to threatened cities.
It often happens that one or two extra defenders (even weak ones that are effectively defensive suicide) can prevent the fall of a city, not by winning but simply by using up an opponents attacks, in order to stall for time just one more turn until the stack of riflemen arrives.
Cats and Trebs fit this role perfectly especially since once collateral damage has been dealt and a few rounds of combat have played out they will end up stronger than, and thus defend before, severely damaged units that are actually more expensive and valuable.
So, flanking units in the attacking stack are one key deciding factor, the other is how much siege they have brought and the potential for my own flanking units to wreak havoc. There are two considerations here...
1. If my own siege could weaken them enough to allow my flanking units a better chance at removing some of their siege then out some of them go!
2. If my own flanking units can succeed without help then I'll hold back until after my flank attacks. An attacking stack with less siege and thus less potential for collateral may then be worth suiciding against because the reduction in attack values may make my remaining defenders strong enough to win if they are not going to get a lot of collateral dealt to them in turn.
So, to sum up...I simply don't see it as a clear cut rule to always suicide siege, I really think it depends on a lot of factors!