I can think of two main issues. The first is simple politics: The fewer LGBT folks there are, the less impetus there is for positive change for LGBTs. For illustration, flip over to race- if someone who is 7/8ths white and 1/8th black identifies as black, they increase the amount of political pull that blacks have and then diversity-related benefits that go to blacks. If they identify as white, they don't significantly increase the amount of political pull that whites have, and there are no diversity-related benefits keyed off the percentage of whites (and the more whites, the less benefits for minorities). Regardless of how they identify, they have the same racial admixture- but their identification has real external impacts.
The situation isn't the same for LGBT, but one of the main similarities is that a person's support for LGBT rights is highly related to the number of LGBT folks they know- and so the more LGBT folks are out there, the more LGBT folks there are to know.
The second is the implications of what a change means. People don't start off identifying as LGBT- they have to choose to identify. To then choose to not identify as that anymore suggests that it's not an immutable characteristic they discovered, but either that it was temporary or that they were mistaken. Either of those alter how it should / will be treated. Suppose some subset of people are "gaight"- they're gay from puberty to, say, their early twenties, then they're straight afterwards. If you can't tell them apart until they're older, then coming out as gay is no longer "this is the real me, cope with it" but "this is the real me, maybe, but you can hold out hope that the straight me will be back."
When I came out as gay, I expected to hear a lot of "it's a phase," but I was surprised by the number of people that told me "it's a phase that I went through," which is relevant information that decreased my expectation that I was going to be gay for all of my life (it's still above 95% that that's the case, but it's less than 100%.).
(This is even worse for transfolk- if gender dysphoria goes away on its own for X% of people that have it, how will that impact the treatment for all people with gender dysphoria?)