This is a problem and a gameplay balance issue. Not only can demomen spam out, but demomen can also spam in. I've seen demo's jump to on top the bridge, then spam in through this hole. The Scottish Resistance works especially well for this.
Dragz, while I'm inclined to agree with you I think you are overlooking two key points:
1) we run an instant respawn server. While I personally find it more challenging to cap the intel on IR, most players ignore the intel and instead play some version of team-deathmatch. The spiral doesn't get a whole lot of action. Enemies who make it into the hay room are more inclined to head for the battlements than they are for the spiral. More easy kills out there than in the empty basement.
2) you're a scout. No one cares what you think. Besides, shouldn't you be chasing people around in the sewers?