Times when it's nice to own a tank:
When you want to dive and the LDS is unreachable
When you call a dive and have to return a tank full of nitrox that you paid for and didnt use
When the limiting financial factor in your diving is tank rentals
When you can't rent the tanks you want to dive
Other than that, it sucks
I have to find somewhere to store them, I have to get them filled, I have to get them inspected, I have to service them. Tank rentals here are $2/day, so thats ten days of diving a year just to break even on the inspection. If you spend $300-$400 on two 3AL tanks, and dive two days every two weekends, it's almost five years for the break-even. However, #1 and #4 mean I own tanks.