I too saw the "straw show" I think is was Fabulous Cakes in Philadelphia?? And the cake that they used them in was about 4 tiers high and the straws seemd to work great. I have used the bubble straws also on my tierd cakes in the past and found that I really liked them too, however, when the girl on the show used hers, she shoved it down into the cake as far as it would go and then slid it back up just enough to be able to genly snip it off and then used the scissors to push it right back down. Seemed really easy. Especially since I have tried pre cutting my bubble straws and rarely get them perfect, then I have tried her technique and, maybe because the bubble ones are so thick, they dont slide back up through the cake as easily.
Anyway, they used them, I think I might try them too. And, besides that, Ill bet you can get them pretty cheap at Costco, etc. (Or just get in the habit of grabbing a few every time you go for burgers...okay, that's just bad. I'm sorry. No I do not support "stealing" straws from Micky D's!!)
Oh, I forgot to add, I DO believe that they ran atleast one if not two long wooden dowels through the entirety of the cake for additional stabiliy.