It seems to me like the primary benefit of typical EA donors (say, most GWWC members, or anyone giving less than $100,000/year, i.e. the vast majority of us) giving effectively comes from the signaling effects of this behavior on helping to promote a culture of effective giving and effective altruism.
It still seems very worthwhile for typical EA donors like me to donate, since the direct value of my donations is still substantial and there's potentially this even greater signaling benefit on top of that.
That said, as Ben Todd summarizes in his answer, most EAs (i.e. everyone not in the reference class of people who have a nontrivial potential to become very wealthy EA donors) can probably do even more good through various kinds of work that help deploy the large amount of EA funding that already exists better and faster than they can through their modest donations.
Given that, I wouldn't want to encourage a small donor to donate a modest amount at the expense of them putting less time/effort/attention into shifting into a very valuable direct work career that helps deploy existing EA funds faster/better. But, if donating some percentage of a person's typical income helps keep them engaged with EA and thinking about important questions related to how we can all do the most good, then it definitely seems worth doing to me.
If anyone thinks I'm wrong about this, please let me know!
I'm curious why you and many EA's who focus on longtermism don't suggest donating to longtermist cause areas (as examples often focuses on Givewell or ACE charities). It seems like if orgs I respect like Open Phil and long term future fund are giving to longtermist areas, then they think that's among the most important things to fund, which confuses me when I then hear longtermists acting like funding is useless on the margin or that we might as well give to GiveWell charities. It gives me a sense that perhaps there's either some contradiction going on, or I'm missing something, but either way it makes it very difficult for me to get others excited about longtermism if they won't enter it with their career and even the die-hard longtermists are saying marginal funding is useless or at least worse than GiveWell charities.