Kodak used to supply chemistry this way for the average user and it was quite convenient. The bag came totally sealed and was punctured by a sharply pointed spigot that was packed with it. There were some minor problems. Once opened, the keeping was not as good as a totally sealed bag, you could not totally empty the bag without some effort, and some of the spigots dripped slowly so you had to place a pan or something below them to catch the drip, and the drip sometimes was delayed starting so you never knew when or if it would begin to happen. I used to use a tray under each one just in case.
The bags were also sold empty OTC but were discontinued about 20 years ago, probably to the above problems. They did not have a good reputation. I have some here that are in storage. I no longer use them.
However, over 20 years, the technology has probably improved enough to eliminate these minor problems. If so, I think it is a fine method.