The one argument I have for giving them the exp is if they are going to neutral portal's grunt path, they're giving you free reign over their grunts on the normal paths, as well as a chance to dmg towers.

But that's really situational i spose

not worth it, imo, except under 1 circumstance, which is if the neutral portal creep waves converge at some point with the normal creep waves so you can knock out both of them at the same time with an AoE attack. 

Very Worth it, as Kitkun said keeping the nemy behind in war score is a priority, also if the nemy does not get hp for towers you creeps might slowly puch into his base, although this is highly unlikely as hp upgrade costs only 600 gold and  in the end you might be giving your oponent aditional places to farm, but considering where must neutral portal are Dgs dont usually farm there.

So in the end, i will always get them is a great way to leave the enemy behind in war score and while they might try to farm faster they will have to move from lane to lane, and as you are supposed to be in one of those you might be able to criple his whole farming operation if he gets to greedy.

I find it depends really. I used to just let them take it, however now I think it's only worth it if you've got excellent anti-creep Demigods in your team - because you can burn through them without losing too much time elsewhere on the map.

I mostly play as Rook or Erebus. When playing Erebus I find it's not worth the additional effort, depsite the speed advantage, however if I go Off-Spec Hammer for Rook it's absolutely worth it. 
It mostly works great if your ahead in War Score but behind in the levels - let them cap the flags and then farm the Creeps to make the difference. If you've got a good team, feign falling back a little as they take the Portals. Let the other side think they've got you on the ropes and then push forward, taking back the portals as quickly as possible. I've found it really throws a spanner in their works.

As long as you can spare the time to type, it's usually not hard to co-ordinate. I can't wait for VOIP though.

Depends what your enemy is doing.

Its' really not so important to keep those sides portals (such as leviathan) when creep waves are still very weak. Upgrade to fortified structures 1 and they basically do nothing. I will occasionally snag the flag back if it's convienant, but if the enemy is adament about holdings those portals, I ignore them and proceed to push up the middle and destroy as many towers early in the game as possible.

Once they've upgraded to priests it's probably a good idea to keep them contained, though.

Divide and conquor, as they say.

I havnt seen that myself, but I havnt been staring at my exp bar when they spawn...

Dare I ask if it is true?

Whoa, that is something that I've completely overlooked.  This is definitely something worth investigating further.

I briefly tested this, and it appears that minotaurs / archers that spawn out of your starting portal(s) DO NOT provide any experience.  Easy to test, sit in base at beginning of game.

I have not done any tests with neutral portals, or higher end creeps (angels, priests, giants, catas), though I suspect the results will be the same.  I suspect the manual is incorrect.

If we're talking about a "normal" game here, then holding all the flags on the map should be the #1 priority. So what if they get a few more archers and soldiers for potentially a little more XP when it increases your own war score significantly?

Remember: the winner is the team that fields catapaults and giants first.