I use it as a backup, I just let pics accumulate in it until it's nearly full,
You might be unaware of it - do note that Flash memory - especially new, high capacity memory chips - are prone to data retention problems.
Old cards might be able to push 10 years or more of data retention, but for the modern ones, it isn't unlikely for the data to be lost within a year or so.
The diminishing retention time is due to the usage of MLC technology and smaller manufacturing process in modern cards. You might notice how no Flash memory manufacturer advertises in their specs sheet their data retention figure anymore.
If you need long-term backup - high capacity Flash memory cards are not the way to do it.