Are "evangelical" and "mainline" becoming largely useless terms?

[I spent the winter writing songs about getting better]

Yes. The issue being primarily not how the terms themselves but how they are used.

First of all I don't really like "evangelical", because all churches are evangelical in the literal sense. They all have outreach programs and accept converts. But if we simply assign it a multiple meaning it still doesn't work well because it seems now "evangelical" and "mainline" are basically being judged only on whether they are OK with gays. Granted that may basically be the only thing many people care about beyond people with theology degrees, but there are a lot of things for classification that are relevant that kind of get tossed out the window. For example Lutheran Church Missouri Synod is often listed as "evangelical" and is on the ARDA listing, but they have very liturgical "high church" services, put a lot of emphasis on infant baptism and the Real Presence. And in addition to that stuff they also reject a lot of stuff conservative evangelicals are crazy about like the Rapture (the only denomination to actually condemn the Left Behind books formally, because condemnation of that stuff isn't a common thing to do in liberal mainline churches unsurprisingly). So they are considered evangelical for basically no reason than that they are against abortion, unambiguously consider homosexuality a sin and don't ordain women. On the other hand ARDA lists the American Baptist Church as mainline.

Reminds me of something I was talking about with TJ in IRC when he said basically all non-denominational Christians are evangelical and I countered with a local non-denominational church that openly describes itself as progressive and LGBT-affirming, to which he said would just be liberal evangelicals. After thinking about it for awhile I realized he was basically right, they practice believer's baptism as opposed to infant baptism, strictly memorialist communion, don't really have a formal "ordination" procedure (even though the lead pastor has a seminary degree...from a conservative Baptist university), are definitely not liturgical, and have a mission statement that is all about things like "awakening people who will embody the love of Jesus Christ to the world" which is certainly evangelical traits. But they also have a gay pastor so I'm sure no one will be describing them as evangelical.

[I spent the winter writing songs about getting better]

It's tough to see a sola scriptura justification for infant baptism.

The other church I mentioned above probably fails with sola fide, since they are basically universalists. But they don't fit as mainline well either...which probably just shows the should be obvious fact that not all Protestant churches will neatly fit into one of two categories.

[I spent the winter writing songs about getting better]

It may be more useful sociologically than religiously. It certainly seems to have some traction when it comes to voting patterns.

Actually that's my point. Liberal churches with sort of an evangelical "template" don't get considered evangelical, and neither do really liturgical conservative ones. For example how many people would hold a No True Scotsman to if a church encouraged its members to oppose a gay marriage ban it can't be evangelical?