All of the mentioned advantages and disadvantages of an accessory EVF are true, except that I also tend to leave my accessory viewfinder attached to my Ricoh GR at all times, so there's no real risk of losing it. But as much as I like having mine attached, I only use it for about 5% of my photos.

I would not buy an EVF for the NEX-5R. Part of this is simple obstinacy; the 5R+EVF costs more than a NEX-6, which has a built-in EVF that avoids the negatives of adding an accessory viewfinder. Clearly the size/cost/something about the NEX-5R appealed to you, so stick with it for its strengths.

While this might not help those of us who need reading glasses, it's quite easy to hold and frame accurately with an LCD: keep your elbows tucked in at your sides and grip the camera with both hands. This is far more solid than the arms-length approach, giving me sharp results in lower-light conditions.

For low-light and typical vacation photos what I do suggest is taking some of the $200+ that you save by not buying an EVF and get a decent pocket tripod. All of the ones that Joby makes are good, but I'm particularly enamoured with their "micro" series. (I own three of them.) These have a very low profile and stay attached to the camera all the time; the few times I take mine off I usually regret it. There's a lot to be said for being able to put the camera on a conveniently-placed object, compose the shot properly (on the LCD, EVFs are unhelpful here) and use the camera's base iso sensitivity.