Kris, I hate to belabor this topic, but I read back through the thread and found where you said the "joists" are 24' long 2x10s every 36" over the 20' length of the garage and the peak of the roof is 5' from the top of the 2x10s. Is this a fair representation?
If it is, there should be about 7 joists down the 20' length and you want to lay some 3/4" plywood across some of those to store your holiday decorations and other small items that don't weight much.
The builder suggests adding a 20' long I-beam down the center below the 2x10 joists.
Tom Tee and Carl suggest that the 2x10s are undersized if you want to add the 3/4" plywood decking for storage.
-- Tom suggests replacing the 2x10 joists with 2x12s, no I-beam. He also suggests doubling the joints on each side if you add a fold-down ladder.
-- Carl suggests forgetting about the storage.
What I get from all this is that everyone seems to be okay with just hanging the layout from the 2x10s, it's your plan to add the plywood decking that is causing the concern. I defer to Tom and Carl regarding the strength of 2x10s over the 24' span. I don't know that the holiday decorations make that much difference, it's the weight of the decking itself that may overload the 2x10s.
I agree with Carl that Tom's solution is less work, assuming the door opening and end wall would need more support. There is a reason why a finished attic costs more and it's not just the decking and drywall, it's the larger joists, headers, etc., needed to support the weight of people and other things.
Now, if you're just going to lay a couple of sheets of plywood across some joists to hold a few items, I don't know if the level of concern would be the same. To my way of thinking, that would be no different than one of those garage storage racks that hang from the joists, though they probably weigh less than a sheet of plywood. People are assuming you're going to add a full 20'x24' deck complete with fold-down ladder for access.