I don't see any reason to practice going slow for BLD.
In speedcubing, the idea is to turn slow enough to allow you to find pieces for the next step. This has no context in BLD cubing since you're never looking and turning at the same time. For memo, as Chris pointed out, the faster you memorize the less time you have to hold the information in memory. But if the issue is turning so fast during execution that you can't track your moves, then yes, indeed slow down. However, this doesn't seem to be a problem with a ton of people.
One issue I have encountered is going so fast during memo that I don't actually memorize the cycles, I just travel through them(especially dangerous for visual memo).