From what I can tell, the turn to 90 degrees from the missile only seems to affect the agility penalty applied to the PH for the type of aspect ratio (head-on, tail-on, oblique, or high-deflection). High-deflection gives no penalty in the agility calculation, while all the others give varying amounts, so it's the best, but there are times when you want to override the automatic evasion logic, such as when you're on a high speed run to get out of the missile's max range and think you can make it before the missile gets to you, or if you're exchanging SARH missiles with the enemy and playing "chicken" to make him turn away first and lose his missile guidance.
Back in the very old days, Microprose's F-19 Stealth Fighter distinguished between pulse, doppler, and pulse-doppler radars, and you had to minimize your chance of detection by flying either directly at or away, or beam-on to various radar sites as appropriate. I believe "notching" is also a major aspect of missile combat in the DCS games. AFAIK though, it's not modeled in CMANO, which is kind of disappointing, as it makes radars more capable in the game than they actually are.