I think the problem is that mech targetting systems are designed to track targets in relation to themselves and are not optimised for spotting.
When you spot, your compy tells you where the target is in relation to yourself. If you move, it doesn't take that into account because 'yourself' is a relative position. Now if you send that raw data to another mech, you'll need to include your positional data in relation to that mech so the firing mech can calculate an absolute position for the enemy to then calculate a relative position for the indirect fire from. And then you account for target movement.
Or as in real life, the spotter has to calculate the target's position (or the position where they think the target is going to be when the rounds start landing) from his own position and ranging info and relay that estimate to the shooter.
C3 does this automatically, but doesn't AFAIK actually predict where a target is going thus still requiring TN rolls.
Depending on which steps of those your fire control system has inbuilt routines for and how efficient they are, it is entirely reasonable that if you use two points to target an enemy from, you take an attacker movement penalty for each moving one.
Even in a C3 system, that step isn't really integrated. You need LOS from both points to gain the benefit, which means you can't fire indirectly. However if you tag the target, you can do it without a spotter, because you made the enemy broadcast his absolute position.
BTW, don't standard rules say that a spotter can't do ANYTHING ELSE (like moving and shooting) in the same turn that they're spotting?
(Also, why does the spellchecker on this site not include the word 'mech'? ;D )
I thought the spell checker came with your browser, not the web site.