My input: When teamwork is necessary and bots are involved, playing co-op with other people is infinitely better than playing by yourself. I can't count the times my AI teammates in left 4 dead would run around in circles on the other side of a corner while I was down and slowly bleeding to death. It's even worse in versus. Whether or not you'll win, or even get out the safe room, is a roll of the dice if you don't have players on your team instead of bots.
In this game though, I totally agree. I mean, if it were already fairly hard to begin with, I'd argue it, but it's already easy enough to kill/survive on the difficulty that says it's for experienced gamers. I more often than not end up with HP profit from drops. If it got much easier, that'd be awful, particularly for players of differing skill levels.
...but... ultimately, it doesn't seem like there's a great exact balance you can strike. Left 4 Dead was made for multiplayer, and it's unbelievably frustrating (at least for me) in single player. Multiplayer balances very well, particularly on Advanced, and is loads of fun... but with bots you need the margin for error of Normal, because it's basically a roll of the random number generator whether their AI will save you before you take damage, or after you take 30 damage from the hunter right next to them. A Valley Without Wind, however, feels more balanced around the single player experience. Thus, people report some multiplayer problems, like not getting any sort of benefit from playing with friends. I kinda prefer it this way, just because I'm not a big fan of the stupid AI kind of frustration.
...and it's totally not because playing versus with my ex resulted in me getting charged by her into a corner, and dying from only that while the bots were just sort of pacing around inside of acidic spit. God, that was annoying. I get too competitive sometimes. =/