Because perhaps a simpler way is a lower overall fee structure and either an inflation or stake structure.
No (or too low) tx fee means there is no (or too low) cost to spamming the transactions.
Security is another concern , but to tell you the truth I don't see it as a large one well not as large as some others do.
There can be such things as " rolling security" this is not dissimilar to an accepted amount of security balanced with use and other factors .
Chaos. Please implement your sometimes secure coin. My popcorn is in the microwave.
A single-rate tx fee is less chaotic.