Years ago, the recommended practice was to mix the toner with HCA. The fact that the HCA would age and prematurely force disposal of the toner was not a particular problem because the mixture was most often used single-shot. I still think there are performance advantages to using the mixture, but today there are other factors that were not as significant in the past that have to be considered.
One is economy, and the general availability of materials. HCA ages quickly and generally cannot be retained for reuse. Fortunately, its relatively cheap. But selenium toner is expensive, and if mixed with water, can be retained and reused until the selenium is depleted. Where I live, there is only one shop that sells chemicals, and its a significant drive to get there do I don't want to go very often. And they don't always have toner in stock (they were out when I was there yesterday). Also, the cost of selenium toner today is significantly greater than it was in the past. So together, these considerations cause me to look for ways to stretch the effectiveness of toner for economic and availability reasons.
The other consideration is that today we are much more sensitive to environmental concerns than was the case 50 years ago, and dumping partially depleted selenium toner down the drain is really no longer acceptable. I want the processes I use in my darkroom to be as benign as possible, and that means that I want to extract every usable molecule of selenium out of the toner before I dump the solution. That means keeping it around for reuse, and that's not compatible with mixing the toner with HCA.
A third consideration may also be that the folks who wrote the books recommending darkroom practices 50 years ago were people like Ansel Adams whose production rate far exceeds anything that I would do today in my darkroom. Considering his prolific output, he could easily have depleted a selenium-HCA mixture long before the HCA component aged to the point that it should have been disposed of. I tend to print much less often - perhaps twice a month if I'm lucky - and that makes a big difference in how I use materials.
So the key point here is that there is not a single, abstract answer of what is right or wrong, but rather a set of considerations that each user should evaluate together to determine what is the best practice for his situation.