Is It a Good Idea to Start at AM Arms of Banks?

Yes and Yes. There's nothing inherently wrong about working at a big bank's AM arm and there will be plenty of opportunities for you to switch should or if you decide to. Unless you have other offers from similarly ranked, high quality independent firms - I would take it and not think about it again until you start thinking about your next career step. 

Starting at a large bank is not a bad idea - despite the mind numbing corporate bureaucracy that you have to deal with, there's a ton of opportunities. Often you can get exposure to a variety of different things, plenty of career opportunities within the bank should you not like what you are doing and plenty of them have good performing, high quality strategies that you will get a lot of good experience on. I'd also suggest to you that all of the experience you get a big bank AM could be extremely valuable to a smaller AM from a business perspective - i.e. helping w/ distribution, relationships, risk/controls, efficiencies, focus on scale/profitability - that some can struggle with. From a business perspective I do think there's value you and it shouldn't be dismissed. 

On these forums what the majority are hyper focused on are the career paths, experiences and potential for 'investment' folks - so we'll call it research analysts, portfolio managers, etc. In that sense banks tend to be more challenging, as AM arms are not purely there to be an Asset Manager - they are simply another business that the bank has, manages and needs to resource - among the other million things you've read on here.