Here, Alexei asks: "What are the benefits for the US in declaring Jerusalem as Israel's capital?"
This is the opposite question:
What are the disadvantages for the US in declaring Jerusalem as Israel's capital?
The disadvantages of the declaration seem to be very small, if any.
The main counterargument is that it could get more difficult now for the US to negotiate with the Palestine side of the conflict, since the decision is seen as giving up the two-state solution and unilaterally supporting Israel instead.
But then, however, there was no two-state solution imminent anyway, the US has lost its credibility as a neutral broker long ago already for the Palestine side, and Donald Trump is viewed by a large part of the world as an irresponsible deal-breaker, not a deal-maker.
So it seems plausible to say that the effects of the declaration will be very limited. How irrelevant such declarations are overall can be seen by the fact that most of the world (apart from most western countries) has recognized Palestine as a state, but that did not change much as well. (In my opinion, this topic is highly overrated and will be forgotten in a few weeks.)
The main disadvantage is that the declaration will destabilize the Middle East.
What else is new, right? Well, not exactly.
We already know Israel has a powerful military, and given Israel's recent reticence toward the U.S., the U.S. can assuage relations without being seen as "caving" to political pressure. Said declaration will allow Israel to take action against Palestinian separatists in ways far beyond the skirmishes which have taken place.
Which would actually strengthen Israel over time, so any reservations about stability in the region may seem tepid. However, consider the larger geopolitical picture:
Switching gears for a moment, the political maneuvers made by China and Russia play a complex role in how Iran chooses to act within their political sphere of influence. Iran was historically part of the Persian empire, and in fact the country's name was changed from Persia to Iran by their own preference. Iran likely sees itself as an ancient empire seeking former glory. It sees the former two nations growing more powerful while the U.S. is mired in controversy. It sees that none of the other NATO countries on their own could decisively suppress Iran militarily. Then we have North Korea, who's getting everyone riled up by doing exactly what Iran wants to do.
So while other situations are taking everyone's attention, Iran has consolidated assets in the region. Aside from being suspected of funding state-sponsored terrorism, Iran has also undermined various NATO and Saudi interests in, perhaps strangely for their size, Qatar, Bahrain, and the UAE.
With a quick glance at a map you'll notice these countries surround the Persian Gulf. This appears to be an attempt to control the Gulf and encroach on Saudi Arabia and Kuwait, both of which are key U.S. allies. Iraq literally looks like a wedge driven into the region by force. Syria's brutal conflict should be a pivotal attempt to dislodge or consolidate that wedge. Again, look at a map, and ask yourself, is that positioning really coincidental? Knowing that the WMD claim by Bush Jr. was false, why did the U.S. attack Iraq? Was there perhaps a larger political game already underway? We can't answer these questions definitively, but the sequence of events in the region seems to point to only a few possible answers.
With all these factors together, we can assess that allowing Israel to merge into a single unified state is a signal that Iran has consolidated enough soft power to be seen as a geopolitical threat. Possible land grabs by a pro-terrorist faction remains a game no NATO nation is willing to play. We can also imply that the "destabilization" in question is not your typical two-party squabbles but a conflict which could potentially embroil the entire Middle East and perhaps allies outside the region.
So the only disadvantage is also the greatest advantage: recognizing Jerusalem as the capital of Israel will make it harder for Iran to consolidate power. And while I'll probably be viewed oddly for suggesting it ... It could also be the start of something very big. Is that perhaps an even bigger disadvantage? Time will tell.
The main disadvantage is that it will be used as an excuse to foment more violence - towards Americans specifically (or, from most Americans' point of view, towards the Jews is also considered a disadvantage).
Granted, there's plenty of other excuses to foment such violence, so effective marginal impact is hard to gauge.
There's no disadvantage to Americans as far as negotiations, as Fateh/PLO has exhibited a history of rejecting almost any deals offered by USA as a negotiator, including much better ones than are now on the table - and from a President widely perceived as much less pro-Israel than current one. Even Obama's clear pro-Palestinian-less-pro-Israel swing failed to bring any progress in their willingness to negotiate; so there's no harm in the swing in the opposite direction, as the problem clearly isn't and never was the perceived bias; otherwise Obama would have had the deal done that Clinton tried to do.
Let's not forget the starting position in any negotiations and how much their failure can be attributed to US declarations about Jerusalem: Hamas still officially denies the existence of State of Israel's right to exist in the first place, and PLO Palestinian national charter officially states as its goals of destruction of Israel and denies Israel's right to exist as well (articles 19, 20, 21, 22). Here's #19:
The partition of Palestine in 1947 and the establishment of Israel is null and void from the very beginning, whatever time has elapsed because it was done contrary to the wish of the people of Palestine and their national right to their homeland and contradicts with the principles embodied in the charter of the UN, the first of which is the right of self-determination