No, you only have to buy Blockland to:
-Use RTB
-Play online
-Have more than 150 bricks
You can use add-ons in the demo version, aside from RTB.
1) You can modify RTB to allow demo users.
2) I'm fairly sure that you can use LAN servers, beats nothing.
3) If you get your hands on V8 or below, you can hack the demo brick count (Not telling how).