You know, the presence of large, powerful states on the border seems to universally lead to the formation of increasingly centralized states. It wasn't just the Arabs; look at the Germans, who formed new peoples (the "All Men" of the Allemani, for instance) in the presence of the Roman Empire. Look at the centralization of Lithuania in the middle ages.
Really, expecting the Arabs not to unite is the unlikely outcome...
On the flip side, it's not like the Romans and Persians were actively fighting the Arabs like they were the Germans. These sort of consistent attacks were what led to centralization.
Right now, the Arabs fit a niche as a dominant class of traders in both Mesopotamia and the Levant (They were able to exploit this niche IOTL to make their conquests there easier). It is against the interests of both the Persians and Romans to allow the Arabs to unite, and they provide funding to different Arab Confederations to fight each other and their superpower counterpart. It wasn't until most of this funding was cut off due to the last Persian-Roman War that any form of unity could develop.
I'd also like to point out that there are several groups that never united whilst living next to powerful Empires, or at least not for a while. Take the Turks, who, although they often formed powerful confederations, rarely had a centralized state, even though they lived next to the Sassanids. Or take the Germans, who never did unite under one state even though they ended up invading Rome. Or the Berbers. Or many others.
So while there are several factors driving towards Arab unity, there are several factors that work against it as well. In my opinion, it's overly deterministic to discount either option.
While that might be true you probably see multiple Arabias since the coastal cities of the Hedjaz, Yemen, and Oman and the nomadic Bedouin were almost completely different societies that only shared a language and founding culture in the same way the Germans were one people at this time. It really was Islam that fully made Arabs one people and capable of creating an empire, and even then the divisions within Arabia eventually led to its collapse.
And I'd like to point out that the Arabs were far from united early on. It was only with massive conquest, which legitimized a united Arab state, that a United Islamic Empire managed to survive. And even then, there was civil war after civil war.
Sorry about the lack of updates to this. I've tried to get back to writing this, but without any time to focus, it's been difficult. Fortunately, I'm working on another update now, and should have it finished tomorrow (when I actually have time to write, it's the first time in a while). If not tomorrow, then Monday. This is a really interesting discussion, regardless. Whether groups will unite or remain divided is something that is very difficult to answer with the absence of principals such as nationalism (well, there's some proto-nationalism, but a large part of that is in places like Rome that are already united) and self-determination.
How dangerous these groups would be is another question entirely. When you get down to it, the Arabs wouldn't normally be a life-threatening power for Rome and Persia, even united. They wouldn't even be on a peer level. There are simply not enough people. Now, the chaos IOTL caused by decades of war and occupation in Mesopotamia and the Levant, combined with a number of other factors, allowed the Arabs to overcome their innate disadvantages. But the idea here is to completely avoid this scenario.
So even if Islam is formed in any way similar to OTL, who's to say it will be successful in any capacity, and not fall apart after a generation or two? What is it about the Arabs that makes them natural conquerors, if anything?