Nigel I think he intends to half wave rectify the incoming AC just failed to mention it.
Space you can justify it any way you want, no improvement in tantalum construction can override the simple fact that if it DOES fail there's a 90% chance of a fire, it will draw more than enough current to create a plasma ball but not enough to cause the breaker to trip before it goes up. Also you're forgetting mains spikes, which will destroy tantalums. Single or sub cycle transients on 120 volt lines can be in the thousands of volt range, say goodbye to your tantalum and yer house.