What's the reasoning behind changing pilot tones in OFDM? Many OFDM wireless protocols (e.g. 802.11, DVB-T2, LTE, etc.) utilize pilot tones to provide channel state information for the receiver. Within the transmission, the pilot tones generally assume a BPSK or QPSK modulation depending on the protocol. 

Why do specifications have the pilot tones' phase follow a known psuedorandom sequence across frequency and time as opposed to utilizing the same phase or a straightforward rotation for simplicity?
Advantage of pilot pseudo-random sequences in standards (with examples are taken from LTE) :


Interference from other sources with different identities (UE ID, Cell ID, ...) are mitigated if detection by correlation operation is used (cell search for example).
Allow including identity in sequence by changing the value of sequence initialization. For example, pilot sequence in LTE is 31-length Gold code which is reinitialized at the start of each OFDM symbol with a value that depends on cell id.