I think you're talking a number of combos, each of which is reasonable and could appear on its own.
1) Cellar + Library. Cellar lets you sift away bad stuff, then redraw to a hand of 7 (and use Library's action-set-aside to not draw your cellars).
2) Library as a COUNTER to Militia. If your opponent(s) are playing Militias, then you want a Library in hand and then it's a benefit for you.
3) Library+bank: large hand size makes Bank super-powered.
I think these are all reasonable, but none are really all that super-good.
1) Cellar+Library takes a little bit of trashing to set up right, or takes a whole lot of cellars and libraries.
2) Library as a counter to Militia is good, but why would your opponents get Militias if Library is around?
3) Library+Bank is good, but unless you have +buy, you'll be spending huge gobs of money on single provinces when you get a bank, and not have enough for even a single province if you don't.
But, given the right situation, I can certainly see myself playing any of those (and I've had quite a few cellar-Library-chapel games IRL when I play just base set!)
I think you've definitely found something reasonable, but I'd be cautious of overgeneralizing how good it is - I suspect that based on what you've said here, it's not much better (if at all?) than just getting Libraries+standard money, unless there are other support cards around. In one of those games where people are getting banks+cellars+militias+libraries, have you tried just getting libraries, and not taking banks over provinces with 8? Assuming it's a province game.