This is such a non-issue. By the time you get to the levels where you can maintain the Hex spell for 8 or 24 hours, the warlock has several good Concentration spells available which are as good as or better than Hex. At most this tactic gives you a single extra spell slot for your first combat of the day (because otherwise you'd cast Hex during your first combat and maintain it the rest of the day).
Either you don't need one of the higher level concentration spells (and thus it's probably not an important fight), or it's a big fight and you'll need something a little more potent than hex. (I like Wall of Fire + Repelling Blast, but there are many other options.)
I've never seen so many people trying to rewrite the rules of the game (either "you can't concentrate through a short rest" or "you can't take a short rest immediately after a long rest (in a different thread)", or whatever the current trendy nerf is) to prevent a single class from get a single extra spell slot for one combat.
That being said - a literal "bag of rats" really isn't optimal. They'll crap everywhere from the stress of being carried around in a bag, they'll smell, they'll chew through the bag, they'll make noise. Just way too much hassle.
If you are going to do it, do it properly. If you are in town, buy a live chicken from the local market and hex it before you have it cooked for breakfast. If you are in the wilderness, be proficient in the cook toolkit (or hire a cook to travel with you) and survival (or ask someone in the party with survival) to trap a small animal for breakfast and do it that way. If neither of those is a viable option in your current environment, suck it up and get on with your day.
I think the real objections to this are more philosophical than rules or balanced based.