WP pins are not necessarily meaningful. All of the on-chip flash for embedded micros that I've seen in the last few years have not needed an external WP pin.
Presumably stock external flash parts are okay, but it's still conceivable that evil circuitry could cause a write, or at least an addressing "mistake".
Are you talking about flash embedded in the mcu, or separate flash chips? The mcu can usually reflash itself, modulo fuses, but I can't recall having seen a standalone flash chip which lacked a write protect (or write-enable) pin. A physical toggle switch on that line seems like a solid protection against NSA style firmware hacking.
Unless you're talking about a pin supplying a required programming voltage, that WE pin is just hooked up to internal gates that suppress writes. There are relatively easy ways to back-door gates (e.g., with secret access patterns or data sequences).
