Hovering over "Buy"... care to sanity check my thinking?!

[JAZZCNC]

These are really nothing more than stepper driver ICs, of which obviously there are a plethora out there with various ratings in terms of voltage, current, stepping capability and safety features. Bolted on to that are the relevant amount of heatsinking, protection/filtering but ultimately they take a low-voltage signal from a microcontroller or parallel port - with appropriate optical isolation - and move the motors as requested. One tick = one unit of movement (in the appropriate direction, of course).

In simplistic terms yes but in reality then theres much more to good drives than this. How it handles resonance is very important and what makes the difference between poor and good working motors. Resonance cripples Steppers and the drive needs to be able to deal with this.
Making your own is an option but not one I'd recommend and I've seen many people take this route only to end up buying drives after much frustration.

You can get ones with direct parallel port drive (so no motion control; driven straight from PC) but then the honus is on the PC as the controller, hence all the requirements in terms of it being able to keep up. Each pin on the parallel port controls a function, which limits you somewhat to the number of axes/functions you can make use of.

Other alternatively you can get drives controlled from a separate motion controller in between the drivers and the PC.

Not exactly correct. You can get drives with built in Pulse engine so they don't need a PC Parallel port or External Motion controller. IE Stand alone
People often get the wrong idea about how Mach3 or Control software works. The control software doesn't create pulses it plots a trajectory data based from G-code and then hands this to Pulse engine which then does the Motion control side of creating pulses.
In standard trim Mach3 uses a Driver to do this function (paralllel port Driver) which then crunches some numbers and outputs the required pulses thru the parallel port to the drives directly (Usualy Via distribution board called a BOB).
If using an External Motion controller then it hands the trajectory data to The Plug-in for that Motion controller. The controller then does all the number crunching and send the pulses to the drives. The difference being External motion controllers can crunch numbers much faster and output much cleaner pulse stream which makes BIG difference to performance and reliablty.

When you guys refer to BOBs (Breakout Boards) you're generally talking about motion controllers, although you can get BOBs that combine both of the above.

It strikes me that keeping them separate gives more flexibility as the drivers need to match the motors; if you get a combo BOB you'd need to be sure that the drivers side of it matched your requirements and they weren't going to change.

No BOB is just a Distribution board for the I/O signals. Often Optoisolated for protection. Commonly you'll have Motion control board and connect to this 1 or more BOB's for Distribution of the signals.
Better Motion controllers Like those from Cslabs provide direct connection and remove the BOB from the picture.

BOB's, or should say Cheap BOB's are a common cause of many troubles with CNC machines and often under estimated to there importance.
They take all the Signals then distribute them to where needed so if the components used are slow or low spec then any signals going out will be degraded or slowed down.
Often you'll see people use External motion controllers with MHZ frequency capabilty and connect to BOB with 100Khz bottle neck components.
Like wise you'll see drives with Max frequency of 200Khz and people using Ex controller with much higher Frequency capabilty connected to 100khz strangling BOB so all that power is totaly wasted.

So what I'm saying is BOB is very important and while cheap works at the slower end it's pointless getting a External Motion controller for the speed and quality pulses and connecting to Cheap nasty BOB's.

A Good machine needs to have a balanced system with no weak link.