so why is it better to have login Id's in a seperate table
That really depends on what you mean by "customer data" (and, to an extent, on what you mean by "database", since SQL-addressed relational databases aren't the only tool in the shed). If you are using a relational database, the object of the game is to
normalize your data, at least to an extent. For the most part, in the applications I write the login table will have the user's login name (often, that's an email address rather than a screen handle), the "nonce" or "salt" for the hashed password, the hash value of the salt-plus-password (as often as not I'll use SHA 512 and encourage pass phrases) and a user ID. The rest of the data goes into other tables, since there may be several simultaneous values (multiple addresses, phone numbers, different names for different purposes, etc.).
In general, any time you look at a category (column) of data and say to yourself "ooh, there could be two of those", it's time to break it out into a different table. There are some things I have seen multiples of often enough that I'll use separate tables right from the get-go rather than waiting for the other shoe to drop -- it's less work to build a battleship than to convert an outboard runabout into one later.