And spence the catalyst give you decent sustain so...
This is also something I could have mentioned. Have you ever been in a fight and someone pops a heal that you didn't realize they had? Well try lvling up during a fight with this thing. The catalyst alone is the real deal sealer for its lvl up sustain. If you really get your lasties you will be able to get one by lvl 7'ish depending on if you have exp boosting things. If you can then stay in lane until lvl 12 then profit.
Last night I went from lvl 10 to lvl 16'ish as morgana in ranked with just: 2 dorans rings, sorc boots and an ROA. I didn't get a chance to b until I had 6k. I was always at decent hp and mana and just getting stupid farm. I ended that game something like 7/3/9 and 240 minion'ish.
Anyway the point is survivability and getting lasties. Even if your opponent b's and comes back with a few items, if they miss lasties and exp they are still behind and lvls add dmg and survivability more than items early game. You can't rely on getting kills to sustain you.
Also, yes everything is situational. As I had mentioned, if you b your first time with enough money to buy a very big item, then do it. The problem is its hard to do that.
Lets say you go for BF sword first B but you get hit hard and need to b or risk dying with like 100g left. You should not stay with around 100hp unless you have a good way to sustain. The life steal from masteries and dorans will not get you there. A good healer perhaps but that still takes time. You should usually just B instead of risking it, maybe pop off a few abilities to get the rest of the wave but stay safe. Then what if you end up b'ing and still need that last 50g to get the bf sword. You can't wait it out, thats way to long. You can't sell your dorans sword. But you can get the claws, scepter and lvl 1 boots.
Most of the time, games tend to fall into this situation. The key is to always come back with something new to bring to the table because otherwise, they just out'ed you out and now the only difference is that they can do it better.
Also, wards win games.