Pros and cons on both sides, but looking at it from a marketing perspective, a single id would be what most consultants would suggest in my opinion.
Cross marketing is a very effective form of marketing, it's what major department stores are built on. Customer comes in looking for a shirt, may not be interested in a TV, but then again they just might or they might know someone that does and so on.
It works the same way online, the idea is to get maximum exposure for your marketing efforts, and that means if you have everything listed under a single id, all your traffic is being directed to one location where they can see everything you have.
You start splitting accounts, your marketing effort just got a lot harder because now while you maybe generating the same amount of traffic, they aren't all seeing what you have available.
Sometimes people treat ebay accounts like pets, they don't want to keep their dog with the cat, and the cat can't be in with the bird and so on. It's a business, and if you are making decisions based on personal taste and not sound marketing then you are running the risk of harming your own business.
This reminds me when having 32 page websites was all the rage, no one wanted to troll through 32 pages of what amounted to garbage. People want quick, and informative information when shopping online, the moment you start making it difficult for them to find what you have, the moment you start making it difficult for yourself!