You don't typically need an MRI in an ER setting except for the problem stated, differentiating between various reasons for a stroke. A CT can do the rest and is way cheaper and simpler to setup and operate.

There are other reasons an MR could be better though, for example no ionizing radiation, so you could argue you could send more patients through it than you would with an ER CT (you would only send patients you have a higher confidence of them requiring it to avoid spamming radiation).