Well, I don't think holes are useful as a general purpose tactic anymore, but I think holing is still useful.
If your defense isn't very strong or the other team's offense is unusually strong, you need to get inside the other team's zone as quickly as possible. A hole made intentionally can force the other team to pull off some of their attackers. In this context, holing is a defensive move.
If two people have just grinding the daylights out of each other, one of them can make a hole and as Lack indicated, the other can take the zone quickly with his speed advantage. This assumes the defense hasn't completely setup yet, but it can work when the defense is setup.
If you have 3 or more attackers, a hole can put two attackers in the zone right away, and as z-man said, they can conquer the zone. More importantly, the attacker closest to the goalie attacks the goalie directly while the other attacker closes the hole behind them to prevent defenders from coming in.
In most other situations, I've found that a hole just brings the round to close faster but doesn't give the holing team an inherent advantage, i.e. the team that was holed has the same chance of winning that they hd before the hole for various reasons that are situation-dependent.
I agree completely with newbie about defending. It's not really the goalie's responsibility to close holes when he has sweepers--that's what sweepers are for. They need to be watching and closing the holes when they appear, or they need to be giving the goalie good straight walls to grind so the goalie can move faster and seal the holes himself. THey also need to be watching in case someone uses a hole because the best defense against a hole that's been used is to go in after the guy. THen you go in with a speed advantage (usually), throw a wall on the attacker's side that's away from the goalie, come back out the hole, and circle the zone as an outer defender. This will usually kill the attacker and defend the zone in a way that gives the goalie a chance to get straightened out afterwards. If your sweepers can do this and they're good at it, then when someone holes you, it's 4 free points, but you have to work for them.