Basically you only need absurd levels of speed compared to your target when you need to use the factor (missile speed/target speed) to boost your crummy chance to hit to something half decent. When your missile is agile enough to hit the target then speed is less critical; the missile only needs to be fast enough to hit the target in the first place.
I'm not really sure about it...
Agility is indeed very useful in the case of anti-missile missiles to get the best possible interception rate in a size-1 package, but speed is more valuable for anti-ship missiles :
- With decent tech, missile speed will be easily far faster than the ships they're supposed to hit.
- the extra speed will makes them more difficult to intercept by ennemy anti-missile defenses.
Note that I'm NOT advocating to set the ASM agility to 0 - My latest ASMs have a 41800 km/s speed and 17 MR.
This gives them a (theorical) 100% interception rate for anything below 6500 km/s speed (and 65% for a 10000 km/s target), but the high speed gives them more than a 2 out of 3 chances of surviving one of my AMMs (40000 km/s & MR30 - 67.2% survival rate).
With my current tech level (MPDs & 64 agi/MS), the usual formula will lead me to build a slow 28800 km/s and MR32 (!) missile, with indeed a nice 92.2% interception rate for a 10000km/s target... but with a survival rate to the same AMMs reduced to 56.3%.
Furthermore, faster missiles leaves less time to react for the target.
[Edit : shameless auto-promotion]
If you're really confused by the missile design window, you might try my multipurpose aurora tool to try some missiles combination without having to waste research on a test engine.
http://aurora2.pentarch.org/index.php/topic,5855.msg60061.html#msg60061This is a old one however...