Of course you can react, under ideal circumstances. But looking at the regular invaded zones in this game the all have a couple things in common. They all have mobs with tons of HP, treacherous terrain (usually water levels), and tend to have the longest clears between bonfires. There are good reasons for this, if you are piling up the wins for token farms. Are you seriously suggesting that if you are playing peekaboo with the casters in the second half of Shrine while trudging through the water that you can just stop mid engagement to cope with an invader? Kinda doubt it and you are more prepared than the bulk of the people playing this game. And even if the victim is unengaged at the time of invasion, the invader can just go sit in the middle of a bunch of enemies and wait. The host can't quit, zone out, or do anything aside from maybe engaging the area boss or waiting for the invader's time limit to expire.
But lets say I get to face an invader on even footing, like say in a fully extinct Dragon Temple or something. The invader is still going to be much more optimized for PvP than you are, especially if you are on NG. They are still going to be slotting PvP spells while you are probably loaded with trash clearing ones. There is almost nothing to be gained by fighting invaders, aside from minimal souls, but if the invader is spamming corrosive urns (and there are a lot like Jin doing this) just to be a dick, I could be out a massive repair bill (the life protection ring will generally break early) along with all my progress. Its almost always better just to slip on a life prot ring and suicide if you get invaded one on one. I know you get your jollies pissing off the carebears, but for most people this game is more about PvE, not a giant griefer circle jerk.
But Angel is right and we are never going to see eye to eye on this, mostly because I am pretty certain we are from different age brackets of gamer and the whole dick punching helpless baby seals for lulz thing doesn't do it for me anymore. I love the coop in the game, could not care less about the PvP aspects, though I do like how it was incorporated into the Rat King zones and Looking Glass battle. Bell zones are meh, but at least they are largely optional.
Playing through as a dedicated mele guy (though I did minor in Faith magic for homeward and other utility) has been a completely different experience. Trash seems loads easier, mostly because armor actually matters against them, but certain bosses have been a lot more difficult (and a couple much easier). The Spider gave me issues for some reason (my caster solo one shotted it the first play through) and Rotten was a lot harder. My biggest gripe is that unless you want to asthetic farm materials, getting a good engame weapon is a lot harder than getting the essential caster spells. It is also kind of dumb how BK GS and BK Halberd are so much better than most of the boss soul weapons. Also the effects of Adaptability are pretty much negligible. You have to dump a pile of points to see anything noticeable and even then its a minor benefit. Any one hander that scales STR at A or S seems to outclass most of the two handers, as well. Poise seems to hit the sweet spot at 70 and mainly for avoiding stunlock.