Steve... all you have said is on target.
One thing about Agencies that I frown upon, is that they are middle men (with inflated pricing to cover the agency fees) and it does not always go smooth as I would have hoped.
I have had some good and some bad experiences with agent managers. I do prefer to speak directly to the Working Band Manager... someone that I can get eye to eye with straight up and not have to deal with bunch of hoopla.
Unless a particular band is on the up-swing of popularity (sought after in the real sense) it is better to have one sharp business savy person within the band to speak towards band business. It is much easier to pay this particular person/musician extra to perform this task than to hire some business gun to do it... there are some hose heads for agents and they can kill a good gig for a top notch band.
There are many many more logistical considerations when it comes to this upper-crust road show band scene (those that use agency management). Incorporation, Copy Write, Advertising and Promo,Sales and Marketing, Stage Plots and Engineering, Back Line, Load In ... and one heck of a comprehension for Business/Entertainment Contracts and declaration of taxes. Unless you are gigging for top dollar and your bookings are a heavy load (three and four times a week or more, with domestic and foreigh travel) you may be better off sticking with a specified band spokes person that can multi-task. There are just a fist full of groups within this genre that place that much control into the hands of an agency, successfully.
This is one of those deals where you have to pick your poison. I've been on both sides - getting gigs and hiring bands. I'd rather deal directly with a band if at all possible, mainly because agents have their own agenda (not neccessarilly bad) but isn't always in the best interest of the band. Overpricing, for instance, or bizarro riders. Things the bands sometimes aren't even aware of. At the other end of the spectrum, there are venues that only deal with agents. One call for all their needs, no billions of promo paks and phone calls.
If someone in a band or a close friend can handle the business end, at least you've got control over things. If you need an agent, take extra time to see what is offered, what is expected of you. I remember (way back in another life) I passed up a real gem of a contract with an agent. He could renew indefinitely. Undefined expenses off of gross. Couldn't perform within a 100 mile area for a year if I wanted out of the contract. A lot of stuff that probably wouldn't hold up in court, but the time and expense would drag ya down anyway.
A lot to weigh. Nothing's easy.