Yes, sorry, I mistyped. The cache, not scratch disk. In operation, it is some of the same for some functions, as it was explained to me by Adobe at PhotoExpo a number of years ago. I have my PS scratch disk on an SSD, but I don't feel it helps that much. Jut now I opened a catalog I had most recently opened a couple days ago. Slow to load.
But arguing in favor of separate catalogs, I see a major performance difference plus I'm not putting all my eggs in one basket, as they say. I have shot digitally since 2000 and exclusively since 2001 when the EOS 1D was introduced. I've had my fair share of lost images, corruption, near total failures. I have no desire to keep tens of thousands or more images in one database/catalog -- even if it backs itself up.
For my work and workflow, I have no desire to keep every one of the 77,000 frames I shot on a recent shoot in a Lightroom catalog. I can, and have, pulled images from shoots and stuck them in folders I use for portfolios. For those jobs, I'd rather using a simple, quick browsing program that allows me to find the images the clients wants me to work on and move them to a RAW Selects folder which I then use ion LR.
My fine art/personal/travel work is different. It only holds about 8,000 images (RAW, TIFF, PSD), and there the cataloging is perfect. It all depends on what you shoot, how you shoot, and what you feel deserves to be a "keeper" and readily available. And for that reason, I do use other RAW programs (ACR, DxO) if I don't want something cataloged. I use Capture 1, but I hate that they have now decided to try to get into the "catalog" scene. Same for On1 Photo RAW, though it's possible to work around the cataloging function in both cases.