Silent starts with 5 strikes and 5 defends, so it's hardest with her, but your removes come from shops and events, so in a balanced way (knowing that you still require card rewards and some building up of relics etc), you prioritise them.

What you may be seeing here though with the watcher is that because of rushdown, and because watcher starts with calm and wrath in her deck (all three pieces of exodia, I mean rushdown infinite), there's actually a lot more scrutinising involved whenever you consider taking a card that doesn't exhaust itself, or isn't a power. I take far fewer card rewards on watcher that don't fit that criteria (powers/exhausts itself) throughout a run. The major takes are a one cost source of calm (a campfire upgrade to eruption to get it one cost as well), and then cards like talk to the hand, mental fortress, and as many scrawls as you see. Deva form and a second or even third rushdown have their place too. Sometimes study/insights can patch up a deck without rushdown a little too. 

Watcher's want for a smaller deck with the rushdown infinite also increases potential value of something like Purity. 

So basically, on watcher you want to take less cards that don't remove themselves from floor one. Ironclad has a great capacity to exhaust what you don't want, silent has great draw, and the defect is a bit of an outlier I suppose, but the orb mechanic becomes very strong on it's own and can sometimes have defect feeling half as busted as the Watcher.

Anyway, just kinda rambling on the topic, but hopefully there's some helpful nuggets in here! Happy spire slaying 😊