I always saw the romance as under the table, as opposed to openly flirting. Most games have the female leads in tight spandex with massive cleavage dangling in your face. Makeup. Thongs.
I like the Alyx character because her sexuality isn’t shoved down our throats. She seems normal, while at the same time she obviously has a thing for Gordon.
As for her involvement in the HL storyline thus far, I see her as being far from useless. It’s not like she does absolutely nothing, she’s carried over masive amounts of data for the Resistance, cleared a path for Gordon, and pointed us in the right direction in terms of our goals.
I can see that some people would have it in for a female character following us around since in HL1 we flew solo the entire time. But I personally would find the gameplay and storyline a tad bit dull if she wasn’t around.