I always got told that it's dangerous because of (variously) the chance of it slipping off the tiller and getting sucked round the prop, slipping off the tiller and tripping someone up, slipping and catching round the tiller so the boat becomes unsteerable and raising the chance of it being useless in an emergancy because it'd catch on the pin if you needed the rope in a hurry; as well as the usual not how it's done, doesn't look right etc etc
From my own personal point of view, it's because of one man on the GU.
(Longish story coming up, look away now if you get bored easily )
I was steering the old working boat Aldgate on the stretch after Braunston tunnel, and had slowed her down to a crawl (as much as Aldgate does crawl) to pass the moored boats. We came past one boat that was obviously just tying up and it had the stern line hung off the tiller pin. The owner looked at Aldgate and gave me a funny look but I didn't think anything of it until about an hour later, when Aldgate had been tied up at Norton Junction and I had wandered to the pub and sat outside.
The guy off the boat we had passed came up to me, and asked why I hadn't "set up that barge in the proper and traditional manner". I said "I'm sorry; the boat's not mine to 'set up', but I can pass on messages to the owner. What do you think Aldgate is missing?"
To which he came back with:
"You weren't storing your rear string {stern line} properly. It should be in a shunk {hank I think he meant} around the tilly stick {tiller pin}"
Because it took me about a week to work out what he was trying to say, I gave hime a blank look and he continued:
"In the old days, the people working long barges {narrowboats} like Aldgate kept the strings around the stick so they could get to them easily. That's why these types of barge have tilly sticks instead of the tube {tiller bar} being welded straight to the tiller. You are letting down your barge by not keeping up the tradition"
By now I was completly lost and had most of the pub watching us, so I said that I couldn't visualise what he was describing and could he give me an example of a boat that showed what he meant
"Mine of course you stupid girl! You passed me today as I was parking my barge! You came speeding past me instead of stopping while I parked"
Penny dropped as to what he was describing, and I told him firstly I didn't appreciate being called 'stupid girl' and that he was talking a load of bollocks about the ropes around the tiller pin.
He started getting ruder and more insulting at this point (the sort of thing that the mods would swoop in and remove if I typed it ) so I finished my coke, ignoring him, and walked away.
I've never seen ropes around the pin in a friendly light since