I never replied to AQ, but I admit I wasn't completely serious with that. It works well for plugins with reasonable number of parameters, or if you divide things to several pages or something, but even with some of my simple plugins I've done some head-banging to figure a layout that works with the ridiculously large knob foot-prints that result (especially if you want the knob itself to be decent size).

For horizontal sliders it doesn't take much extra space obvious, if you just put it right above or below the slider (basically just the font-height). For buttons it forces certain width, and for everything else it tends to pretty much double the space required. Unfortunately horizontal sliders are my least-favourite GUI control for various other reasons.

That said, I do personally dislike classic popup tooltips a lot and much prefer some fixed status area (eg most apps have one on bottom, many plugins have a fake display or something). The most important reasons would be that tooltips are kinda distracting, especially if they pop without a delay (but such a delay makes them even more annoying when you have to actually hover and WAIT). On top of that they are almost invariably butt-ugly.

There are some exceptions though; for example IL Parametric EQ2 has essentially "tooltips" when you hover on top of a curve handle (or drag it's controls) but they are just text drawn below the control. It'd look messy if that was visible for every handle all the time (they'd overlap and whatever), but integrates nicely to rest of the GUI.

That said, the are also controls where it's just useless. FL mixer (in the classic small view) has a single "LED" per channel on top of unlabeled pan knob and channel fader (these still show changes on the status area obviously when you drag them). You can click the LED to mute tracks and right click to solo. It's brilliant; you can have dozens of channels visible (my normal layout has 45 tracks visible at all times, plus the sends that I never use) and I'd never want them to sacrifice a single pixel for labels there.

Bottom line is: use the plugin and figure out what the workflow needs. If accurate values are important for some particular plugin (or subset of it's controls), then put them visible. If you got the space budget, go for it. It's not so critical though, since most controls are tuned by ear anyway.

edit: oh and my good old Pink synth doesn't even have labels for control names