Personally I can get by easily with just the free retainers and still have almost 200 slots total still free between them, even with all 50s. Depends if you want the clear advantage on the markets really, and the convenience of having more shards and mats gathered. As convenient as it might be it's just an extra service in an MMO, personally I'd rather buy more real games and bundles than rent an extra monthly service on top of a sub, nothing is really worth spending extra in any MMO no matter how much it's tried to be justified and have gladly never done so, but that's up to you to decide really.