Rent vs own is an age-old debate. There are valid business reasons for both use cases.
MT service bureaus (Google, Microsoft, KantanMT & others) charge a fee for professionals to use their services. You are rent their MT engines. As long as you pay the recurring fees, you can use their service.
Conversely, the services stop and if you don’t pay your subscription. If you customized an engine on their service, it’s reserved for you to use but you might lose it after your subscription lapses.
You own you Slate license in perpetuity. You don’t pay a subscriptions or per-word fees or any other kind of fees to use your engine. Furthermore, you own the SlateMT engines you create. They are your property, without oversight and without hidden costs.
SlateMT includes tools for you to backup and copy your engines to other systems. You can share your engine colleagues who can use them on their machines. If you stop using your engine for a year, no one deletes it.