This style of multiple pathing is the most undesirable to me as it promotes cap fests.
Only if you're playing with the crappy old server settings.
Anyway, I think a map is only as good as its tightest chokepoint. If the entire team sits and sprays the one single path that's necessary at some point to get to their base, gameplay comes to a complete standstill. Often times the chokepoint is the base itself. Take shazam, for example. It has two paths, but they both funnel into the base at the same point. It's fairly trivial for the defense to just spray that entrance and make it extremely difficult to get to the flag. So many maps tuck the flag in the corner of a room or something where basically the only way to get through to it is to get lucky running through spray or entirely eliminate the defense. A team can get a small lead and then just "camp" the flag for the rest of the game, making it impossible for the other team to get any significant point gain. Strategy, yes, but I don't think it's really that fun for either team. Even a map like ub_rooftop with a couple different paths into the base suffers the same problem: it's basically impossible to get in, grab the flag, and get out without being eliminated or eliminating the defense.
Personally, I think flags should be very easy to obtain. They shouldn't be shoved off in a corner, or in a small room, or on the top of a big pile of boxes that you have to jump up, or up on some platform that you have to use a trick jump to get to. You should be able to sprint through the base, grab the flag, and get out without skipping a beat (or a strafe-jump, as the case may be). Flags make the gameplay interesting. Sure, elimination is fun, but nothing gets your adrenaline pumping like grabbing the enemy flag, or when an enemy grabs yours. I like for that to happen as much as possible. I'm not saying defense should be impossible, but merely that a single player shouldn't be able to defend the entire base by exploiting poor map design.
The other big thing in pathing and gameplay flow is countering impassible points. In other words, if there are points along the path where a person can sit and spray, making the path virtually impassible, it severely restricts the flow of the gameplay. There are often simple things that can be done to work around that. For example, if somebody hides behind a box and sprays a narrow path, you can put a small box next to the box that an offensive player can jump on, then shoot down at the sprayer player. Often times these problem areas aren't noticed without thorough testing. That's why we've been promoting the release of beta maps. Even if all the textures are aligned and you think you've thought of everything as far as gameplay goes, there will almost undoubtedly be some kind of kink in the gameplay that makes it undesirable.
I think the best kind of map is one that has lots of paths with frequent overlaps. I really like the new map hobo is working on (routez). There are paths up the wazoo, but there's also a lot of overlap, so the paths aren't necessarily distinct paths, but instead offer all kinds of ways to outsmart your enemy. There's plenty of windows and things so you can shoot at people who are on different paths, too. Also, things like having the jail in the enemy base prevents defense from becoming too excessive, and you can get to the flag from the high route, going up steps, jumping up the ledges on the side, going up the ramp, going up the ladder, etc. The gameplay is always moving. You always have to watch your back. It also seems to play pretty well with a small number of people or a large number of people. I hope more maps start following this model.
Oh, and for the record, I think barrels is a terrible spray map. Not as bad as fort20 (GT needs to be taken out back and shot for picking that in a match), but still very bad.