Inventory is at a premium

[Saccharin]

There's been quite a number of threads explaining this already which explains that saving the data to the back up servers is roughly 15(?) seconds at the moment in case of rollbacks, crashes, etc (was mentioned in the Liveletter a week or two ago).

Having more inventory/retainers would slow that process and hence they made extra retainers for 'premium' to deter people from possessing too many retainers.

No, it has nothing to do with backup servers. The data in memory is written to disk every so often so if the server was to crash the data on the disk would be recoverable and you would only lose data between that small gap.

Since data for retainers that are inactive is not needed then there's no writing of data for those ones. Each retainer has a set amount of spaces no matter how many you have so adding additional retainers will not increase the amount of data written to the disk.

They sell them because people will pay for them. This is the shady part of the game where these things are sold as optional but retainers give a large advantage with ventures and also the much needed additional storage space.

[PotatoWafflez]

No, it has nothing to do with backup servers. The data in memory is written to disk every so often so if the server was to crash the data on the disk would be recoverable and you would only lose data between that small gap.

Since data for retainers that are inactive is not needed then there's no writing of data for those ones. Each retainer has a set amount of spaces no matter how many you have so adding additional retainers will not increase the amount of data written to the disk.

They sell them because people will pay for them. This is the shady part of the game where these things are sold as optional but retainers give a large advantage with ventures and also the much needed additional storage space.

Having 8 retainers I will tell you getting the 4 new ones up to max level isn't that fun and managing them all is a hassle with all the going in and out of menus. I will say though that if you have 8 battleclass retainers at level 60 with the right ilvl you will have an advantage on selling monster drops just because of the low drop rates, but I see this as a good thing for the economy more than a bad thing because easy supply is good when there is clearly a demand due to the low drop rates, but it is still very far from pay to win and venture minions drop much easier now as well.