The price of tank rentals is a significant proportion of what I have spent on local diving this first year. I thought about buying tanks in an attempt to reduce costs in the long-term (i.e. several years) but the hassles, maintenance, and filling costs that you and others have mentioned were one deterrent. Another other reason is that it would only make sense financially if I stayed here diving locally in the longer term, i.e. several years, and I am not sure yet how long I will be living here. The final reason is that I would need a lot of them to accommodate my weekend dive schedule, since I do back-to-back trips on both mornings and afternoons on both Saturdays and Sundays. So it makes sense for me to just keep renting, at least for now.
I have noticed that a large proportion of locals on the boats here, at least half or more, bring their own tanks.