An official relay box that sources its own power is indeed a big deal and should always be used; tying to the vehicle's brake light circuits is a very bad idea (other than as triggers, of course.)
The 7-pin (US standard; its different overseas) allows for both a brake controller and charge. Trailer brakes are required in many states over a certain weight and are a darn good idea with a lighter, shorter-wheelbase vehicle whether required by law or not. Without the charging connector the breakaway battery has to be manually maintained and if you don't, and it is dead when needed that's very, very bad. Any time you have electric brakes on a trailer IMHO having the charge connection hooked up (properly fused of course) so the breakaway battery is charged whenever you're connected and towing should not be considered optional.