For towns, a responsible mayor would keep 2-4 of the animals in a safe place where they could be bred again to restock. There really is no need for spawners.
For the sake of information I'm going to point out the differences in Spawners VS Dynamic spawn and this won't be a short post:
1)
We don't know.
This was going to be point 2 but I think it's the most important one: we don't know the rules.
What's the limit? What's the area? No numbers, no parameters, no nothing.
This is bothering enough people to cause multiple threads to pop up.
We don't know how this system actually works.
To have a proper idea of what's going on we need to know the mob limit per player, mob limit per area, total mobs on the server at once and the despawn threshold.
I've been asking this even before the server launched.
Those informations would clear any doubt without the need for further discussion over conjectures and extrapolations. We could set town rules, design stuff and have less problems for everyone.
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2)
The main flaw: despawning, because "Dynamic" is not "Incremental".
Since I've seen it happening right in front of me, those 2-4 safe animals could despawn.
I'm assuming this can still happen since I havent' seen any announcement regarding changes to this system.If they CAN'T despawn, then the system isn't dynamic at all and it will be clogged up.
A mayor can keep safe reserves, but it's impossible to control how much people will be breeding, luring, etc.
There must be a limit and when it's reached the system should redistribute the mobs depending on how many players are in a certain zone to keep things balanced.
Redistribution could cause "safe reserves" to disappear.
This is how it was advertised to work and the proper way to avoid having problems when getting new mobs.
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Now think about this worst case scenario:
If you have those 2-4 safe animals, you'll never kill them because you need them if something bad happens.
If more and more players are doing the same, what do you think will happen?
Eventually some of those mobs must go away, and it won't be pretty for the poor guy who's left with nothing.
A spawner is NOT a mob. 2 "parents" are indeed mobs.
Same worst case scenario with dynamic spawning VS spawners:
50 players in your town are keeping 2 animals of each kind as a safe reserve.
The limit is reached (I'm just making numbers up, but it's possible) and nobody will kill any of those animals because of course they need them.
Then you try to breed something and find out you can't.
Then, if the system is doing its job, a despawn will trigger and someone will be out of "parents".
With spawners:
Same 50 players but there's no need for safe reserves.
The limit is reached anyway (because it's possible) and the server triggers an automatic despawn.
Let's say 50% of the total mobs, but even 100% would be fine.
Any problems? Not really, because spawners aren't mobs and they will start to spawn mobs again.
For the same reason, spawners will never count towards any mob limit and the spawn rate can be fine tuned as needed.
Even if I'd prefer a more natural spawning system, I can't see how this could have any edge in terms of efficiency and stability over a seemingly way more solid solution.