If the spotter is killed you'll get a message: "Spotter has no LOS" and the rounds won't fall so accurately. (It's difficult to see things clearly from Valhalla.) The last spotter who assigns or shifts the fire mission or airstrike is the one who determines LOS to the target and therefore accuracy IIRC.
Yes, technically speaking it can be inconvenient to keep track of multiple spotters. Practically speaking though most battles it hardly comes up; usually it's either the HQ or dedicated FO that does most or all of the spotting. If you're keen on using combat "0" units to adjust fire then it's more complex, but even so you can shift between Bombardment and Battle Screen to check which leaders have LOS to a particular target and which artillery gun, battery or aircraft has a nearby plot.
Most games I don't bother with frontline leaders in that role--putting them in view of the enemy can be deleterious to their health--and my spotter (HQ or FO) is tucked away safely in the rear. Just about all bombardment missions will have lower accuracy, but that doesn't seem to affect gameplay or overall results too much. I've had plenty of kills even without LOS, so why risk valuable units in pursuit of perfection?
Incidentally there's a scenario (#195--Bringers of Punishment) in SP Main Battle Tank that gives the Israelis four high-value FO's in well-placed vantage points around a Syrian town. That was one of the few times I've fine-tuned spotting assignments for a battle, not only assigning airstrikes but also altering flight paths with each spotter to minimize flak and SAM exposure for each mission. Nor was re-plotting all that difficult; once initial missions were completed it was easy enough to see which spotters had LOS to remaining targets. The attention to detail paid off with max damage inflicted for relatively modest losses. You can read about it here if you're interested:
http://forum.shrapnelgames.com/showthread.php?t=50111
Hope this has been helpful. Cheers and happy gaming!