Just couple of side notes in addition to all of the above:
- your combat toon cannot prevent attacks on your miner completely. At least not on his own - enemies do indeed spawn fast enough to prevent one toon from keeping an area safe. Multiple combateers, however, keeping enemies busy, could reduce the chance of miners being disturbed to almost zero -> teamplay possibilities
- while patrolling the corresponding combat area, your combateer has a higher chance of "intercepting" enemies than if he's idling in the mining field
- if your miner is pulled into combat by an enemy, you can attack that enemy with your combateer and most probably make the enemy change his mind. As soon as no enemy is targeting the miner, he can leave the field immediately, w/o waiting for retreat counter
All in all, think about it in the following way: You're going into a low-mid LL zone with 2 toons. Enemy density is rather higher as is their aggressiveness. Your miner, well, starts mining precious ores right under their noses. What can you combateer do vs. say 5 bored enemies? Obviously, he cannot block all of them at all times.
1) Your own combateer (1x) and your own miner (1x) were never mean to be a constellation that would allow you to mine at no risk and w/o being disturbed in low LL areas. Having a combateer around (either your own or a friend) does make it way more comfortable for you and decreases risks, but, obviously, shouldn't allow "afk mining".
2) For larger groups of players, however, the situation is very different: Multiple combateers (appropriately leveled) should be able to keep enemies in a given field busy most of the time, allowing way more miners to do their job almost undisturbed. That is, again, if combat field is kept busy by few fighters, any number of miners can be mining rare ores (somewhat balanced by the fact that amount of high level ores is very limited).