Sure, but unless you're using them for training, the power usage for inference will vary a lot. And it's cumbersome to shutdown the instance while you're working on something else, and have to start it back up when you need to use it again. During that time, the vast.ai host could disappear.
Most people don't think of storage costs and network bandwidth. I have about 2tb of local models. What's the cost of storing this in the cloud? If I decide not to store them in the cloud, I have to transfer them in anytime I want to run experiments. Build your own rig so you can run experiments daily. This is a budget rig and you can even build cheaper.
Let me add that moving data in and out of vast.ai is extremely painful. I might be overprivileged with a 1000 MBit line but these vast.ai instances have highly variable bandwidth in my experience; plus even when advertising good speeds I'm sometimes doing transfers in the 10-100 KiB/s range.
