I don't tend to complete a campaign either. Aside from perhaps wanting to see it through to the bitter end, it's all but over after taking 30 provences or so. Heck, really you know you're on the path to victory once you have secured your borders and weakened your neighbouring factions. The best part of any campaign is probably the opening 20-30 years, so I'd sooner start over once my faction is the obvious super-power in the game and enjoy a fresh challenge, than just continue the long and tedious 'clean-up' operation that is to attempt 50 regions or total world domination. I suppose with BI you at least got loyalty and hordes to add a continuing challenge throughout the campaign (although at times those things don't add up to much), but standard RTW has nothing to warrant playing on for more than necessary as far as I'm concerned.