First things first, you are right on that Q3's spawn system was not random.

Now in QL the rule is "any of the spawns further from the enemy's current position".
Further = half of the spawn points in the map.
Current position = the killed player could have been fragged anywhere in the map, or the killer been anywhere when that happened, and still that doesn't matter at all. The only thing that matters is where the enemy is right when you spawn. That wasn't the case back in Q3.

So, in QL there's 8 spawns in ztn, then 4 are possible given a position. From the possible spawns any of those 4 can be where the opponent spawns and from a same position the possible spawn points are always the same. In T7 there's 16 spawn points total so 8 spawns are possible (one in eight chance of proper spawn rape) and in dm13 it's 18 total so 9 possible (one in nine).

Here's an useful tool for those things:
http://www.esreality.com/?a=post&id=2219012

Also, QL's spawn system is not the only thing that has changed. What they did was also changing spawn points themselves in popular maps, so the rule is better applied in practice.

After all that, then they tried to remove weird initial spawn combinations, for example grenade spawn in ztn against shotgun (yesterday I re-watched noctis vs. rapha in some ESL event in 2009 and noctis lost all the monies because that wasn't removed yet, lol).

So with all that said, the system right now is quite different from Q3. Now spawns are mostly based in areas, like in T7 and aerowalk for example, which makes spawn killing way harder than what it used to be. At the same time that adds a new layer of skill, which is that the killer can choose to force a spawn further from him (useful in dm6) and the killed opponent too.

All that makes QL's spawns highly predictable (I agree with you) and knowing about spawns is a skill to master. This is not just pew pew everything anymore, just running around the map 100% randomly shooting at spawn points, like in QW for example.

From a players' perspective, that means that at first everyone learns the sweet spots first, then improve into having more or less an idea of where the opponent might be even if the sweet spot fails, and finally you just know all the possibilities at all times.

So in that perspective, a good ztn player knows all spawns possible at all times (the four positions), so if I don't see the opponent at the spawn I'm forcing for killing I still know 100% surely or at least 50% exactly where he is. I'd call that highly predictable :P.
A good T7 player knows the areas rule of that map (RA and mega), so he can at least assume that if the opponent didn't spawn at RA then he is at mega. A better player knows all the spawns that can be forced from your position, even if they are in your same area, and exactly the part of the map he is if he hasn't seen him (100%. That includes bottom YA area first and second floor). Even in dm13, which is the hardest of them all, a good player knows the sweet spots to force around 2 or 3 in his point of view and still knows all the possibilities (the 9 positions). In dm13 the freshly spawned player isn't as fucked up as usual, if not immediately spawned killed at top mid for example, but still is.

These are things that only a good player can understand and apply them in-game, while the rest will just say that the game is random and cry. BTW just reading this is not enough: it's way better to know all that from experience, so actually know how it works in each scenario. No 1200 elo crap.

Some maps are kind of broken though, like hektik, and others are never perfected, like dm6 (lg and YA initial combo. The very worst spawn in the game) and T7 (some spawns can be forced no matter the area). That goes to show that either the devs stopped caring or that they aren't skilled enough at their own game to actually understand how this works in practice, lol.

Finally, all 'pros' know about this and master it in all maps. They not only know this from experience, but also know the technical crap. I still remember talking with strenx about super pro possibilities and alternatives of forcing spawns in aerowalk in QL: not only he has it nailed to the millimeter, which actually matters in aerowalk, but he has thought about and practiced those possibilities too, so for the latter he's beyond me :P

And to nerd it out:
a) Very rarely some spawns just are fucked up. I still remember a cooller match in ztn in which he was jumping to RA and the opponent spawned in rail.
b) I was just re-watching one Memento video and he says the 'further distance' rule is computed only using a 2d map, so horizontal distance only Vertical doesn't matter, so in the said cooller example the explanation shouldn't be that he was touching the ceiling or something. The system just fucked up, which again is something that almost never happens.