I would deffinetly recoment getting it. Barbarian Invasion only cost $30 (that's when it first came out, now it may be lower), so it's deffinetly worth it.

Also, as you play RTW and BI and all, you'll get a bit bored sometimes. This is where the great mods for RTW come in. For most new one's you will need BI, not to mention BI is a ton of fun anyway! I mean you can play as hordes!!!