These are all pretty minor and unlikely concerns when stacked up to the annoyance of having to carry around something of so little value.
Viz, it more than doubles the time taken.
How many seconds are we talking here? And how many extra seconds are spent doing something else far more annoying, carrying it around? You'd have to fasten it to your bag, or put it inside and then get around it every time you want to get something out. Or you take it to your locker or workstation, in which case you have to remember to go get it before it's time to go. You'll probably drop it at some point, possibly damaging it, or someone will sit on it or stand on it. Really, it's just an arse that I don't see any point in doing more than the barest minimum with.
There's also a lot more valuable and important parts of my bike I'd rather some arsehole didn't spew in or piss in. At least discarding a dirty helmet is a seconds work. If they spewed on my seat or in my rear cog set, it would be a hell of a lot more trouble, and if they want to actively vandalize a bike they can ruin it in seconds. It won't be my shitty helmet I'm cursing about if that happens.
Sorry about a little bit of the language, guys.
Completely forgot to say no worries and an apology like that shows a lot of class. I definitely agree that some cyclists are jerks. But I have felt a whole lot less bitter towards lycra cyclists in the last year or so, somehow their getting in the way just melted back into the perspective of all the other things that cause trouble on the road, on which list they came up somewhere around 50th place. Even a big bolshy pack hogging the whole road are only going to be in the way for less time than a whole light phase, and all they mean is you have to slow down to like 35km/h for that whole time!! The horror!
Maybe I'm mellowing as I age.
here's an interesting post making a point I've made before: for men, one way to shed some privilege is to get on a bicycle. Suddenly it isn't all about you and if you do get hurt it's going to be your fault (even if it isn't). Andrew Geddis made the point even more clear when I suggested he try it by saying "but I can't ride to work, it's too far and I have things to do". Yes, exactly, it isn't all about you. The transport system officially allows you to cycle, but don't get above your station and assume it should be possible let alone easy to live a normal life and ride a bike. Similarly, ... and be a woman.