Instead of an "instead of" trigger, you should use a "for" or "after" trigger (they're the same thing).

Another option is, if the inserts into the main table are handled by a stored procedure, have the stored proc insert into both tables when appropriate, and don't use a trigger.

The advantage to triggers is they are difficult to bypass. Doesn't matter which proc you use, doesn't matter which front end application you use, the trigger fires.

The main disadvantage to triggers is they are easy to forget about. The business rules change, and the trigger keeps on enforcing the old rule, because nobody remembers it's there.

I've found them useful in certain, limited circumstances. Mainly for complex referential integrity issues.