Having a small flyer, I wanted a large flyer, and 4U 3/3 upside seems to be acceptable if not the most common blue common flier.

Definitely acceptable, indeed: Shinen of Flight's Wings was one of many such.

Seems overcosted if anything - one extra colourless for an untap ability to expensive to use very often.

But Phantom Monster doesn't happen at common these days. It barely happens at uncommon.

Yeah. There's obviously better cards for constructed, but in limited blue should be grateful to get a 3/3 flyer AT ALL. It normally gets a 3/2 or several 2/2 or similar. But sometimes a 5-mana 3/3 with random upside to try to make it relevant for something. If you're looking for an aggressive deck, 3/2 3U or 2/2 2U is better, but in limited sometimes you want 3/3 and waiting an extra turn isn't the end of the world.

I wanted a 3/3 so I picked a small benefit that would rarely be relevant, but might be sometimes.

In fact, I'm embarrassed that this card was printed years ago with 2U untap. There just isn't that much design space. But I think it's fine.