ISP BGP point-to-point links between routers?

Does anyone here know how best to handle these? Specifically the /30ish subnet between the routers? All the BGP guides I find are somewhat generic and always discourage advertising these into BGP itself but never explain why. They mention using an IGP, loopback & static routes, and I do understand how all of that works; but what is recommended in real life scenarios?

I know you can use an IGP to handle this but I am interested in how ISPs connect their iBGP routers.

For example, one datacenter I manage has a BGP session with 2 different ISPs to 2 of my routers. Cogent to router A & Hurricane Electric to router B. They each gave me a small public subnet to peer with them on. These subnets are advertised because they are routable on the Internet. But beyond that I have no idea what ISPs are using to connect routers in their own AS and to external ISPs.

A part 2 to this question would be how ISPs interconnect with each other. I know generally they converge at Internet exchanges with a route reflector/server, but I am interested in the subnets they are using to do this. Are they public and Internet routable?

One last thing. I have 2 different routers, one connected to each ISP (eBGP) and then they were connected to each other (iBGP). They are each getting a partial/default route table from their respective ISP. Instead of using next-hop-self, I simply advertised the /30 into iBGP at each router so they each have an organic route to the next hop ISP router instead of router A announcing itself as the next hop to router B for routes on Cogent. Is there anything wrong with this?

Thanks for all who chime in ;)