Unless you are a techie, or the guy everyone goes to in the team to tech them, then you can get every manual you require for your/the teams markers before going to the site
If someone comes to you for some random manual and a connections not available then tough, they should have thought about it/packed their manual or a copy of relevant pages
For iBooks you don't need a live connection on site - load it into iBooks and then its ready when you're on site
You can buy a iPhone marker manual app, but even at 69p why pay for something available free
PDF manuals are all over the Internet on manufacturer websites and lists of links on forums
For free you can have your manual in moments or a selection of most markers built in 5 or 10 minutes
I tech my whole teams markers, thats 3 different brands, 7 different models, 3 different marker types completely. Most of them have been brought secondhand so simply remembering to bring manuals is frankly not the issue as many didn't come with manuals. (Copy/printouts are just a pain to keep neat and readable in a paint ridden environment.)
Due to this, a FREE app that stores all my needed pdf marker manuals in one easily readable place without having to search for the correct manual edition on various forums is perfect for my situation. Not all of us have iPhones.
So overall to be honest, your whole argument against it kinda kills itself when you try to say iBooks is better because it's free. This app is free...
Oh and sorry if this came across rude, but this app is perfect for my situation and frankly your argument just didn't make sense to me ;D