You’re right. I didn’t realy assume, that the “on attack” skills proc on Sorceress-casts. But I wasn’t sure. I think it’s “on attack” with the physical part of the weapon. You don’t need to hit, though. (But I’m also not sure about that. But if you need to hit, then again, what’s the difference between “on attack” and “on striking”…?)
About the Synergies: I never got so deep into the game. (I reached hell, act 3. back in time. But only in SP via an item-find mod with my assassin. I had full Natalyas Set, then, But I gave up there.) But I think they were intended to get Pre-requeset-skills more used in early levels. At least this is, what I read. Now, Players tend to min-max in such games. So nowadays, you max an endskill, and then try to max the synergies. I don’t like this. And I will try other ways.
I play a Sorceress, for example, at the moment. And I got through the game pretty well, so far. (I’m in Act 2, NM) The skills, I use are chain lightning and Frozen Orb. I skilled Frostbolt in early levels (a synergy to the orb, the only one) But only, becaus I used that skill, and I assume, they’re not realy NECESSARY to get through hell solo. (E. G. a “players 1” game.) I didn’t synergize the chain-lightning, yet. But I think I will only get 20 points split between some of the skills, that synergize chain lightning, so I might also use them, now and then. (Nova comes to mind. Doesn’t have a synergy on its own.) My main attack is the orb. But since it has a casting-delay, chain-lightning is the perfect skill to use until the orb is ready again. I will also skill blizzard a bit, I think. I throw this still in, now and then. I won’t put more than the point I already spent in Thunder storm, for example. Imho, this is a perfect example, for a one-point-wonder, and every sorceres, who intends to use damaging lightning should get it, imho. Also Energy-shield. This can also be a one-point-wonder. Although, you need charged bolt and lightning for this… (and, you still should priorize vitality, non the less, Except you max this skill, including telekinesis. But I think, this is a waste of 40 points better spend in damaging skills… For pvp, this might be considerable, however, in conjunction with thunder storm… But I don’t play pvp, especially not in diablo 2…)
To the crossbow for the necro: I just saw, there is even a unique one for him… (with +skills for the necromancer and several poison procs when struck or striking…) So this might be exotic, but not impossible… ^^