In a perfect work, you would use RAID 10 for everything. However we don’t work in the perfect world, we have budgets to deal with. And these budgets mean that we have to make sacrifices at times so we don’t get what we want.
RAID 10 is very expensive to implement, much more so per gig than RAID 5 or RAID 6, especially if there are a lot of disks in the RAID array. If the database doesn’t specifically need more performance than the RAID 5 array can provide, then using a RAID 10 array is just a waste of money. And that money is money that could be used for other projects that the company is trying to complete.
In the real world that we all work in (or at least most of us) performance comes at a cost, and those costs have to be controlled. If you need RAID 10, and you actually need it, and you’ve got the budget for it, then use it. Otherwise use something less expensive. If you aren’t sure if you need RAID 10 or not, start with a lower level such as RAID 5 or RAID 6, and if needed switch up to RAID 10.