Small example:
Let’s say we have a site with no good cheap access (FiOS, business class cable, etc) so we get them the following circuits:
2 6Mb/768kb DSL circuits. (and where I am, that 768kb is often overstated. at one of my sites I get 384kbps on a great day)
1 2Mb/2Mb Ethernet circuit.
Now, it’s not necessarily an improvement on things to bond the 2Mb circuit with what is in effect a 6Mb/384kbps DSL circuit. In fact, I’ve seen that this negatively impacts the SpeedFusion up link performance (no fault of SpeedFusion, I’m sure, just the up link on many of these asymmetric DSL circuits is pretty rough)
So, I’m left with using the 6Mb DSL circuits for general internet access and the 2Mb Ethernet circuit for site-to-site VPN.
If I had a little more control over routing, I would instead:
Asymmetrically bond a 2Mb Ethernet circuit with a 6Mb DSL circuit, using both the DSL and Ethernet circuits for down stream site-to-site traffic and only the 2Mb Ethernet for out bound site-to-site traffic, allowing me to take advantage of the down link performance of the DSL line without having to worry about how crummy its up link is.