I posted this on the GG forums and on the HC forums... I still stand behind my belief, although I think that tap tempo is a nifty feature to have regardless of how practical it actually is.
I meant to post this a long time ago, but it came to me again while taking a shower, and I want to say my peice before I forget it.
OK, first, let's consider tap tempo echo. I'm sure many of us have come across it... the DL4 is probably the most commonly used of tap tempo delays, it's also the one that I've used the most, so I'll use it as the example. I believe it's safe to say that all tap tempo delays are digital, but I'm unsure-however I know that tap tempo can only really be done with digital components. Anyways, let's start with a scenario.
You're playing with a drummer who can keep a decent beat. You want to add some echo to make your sound more "full". But, you want longer repeats, and want them to be in time. So you put your DL4 on analog echo mode, and tap into the drummers beat. You set the repeats to about 3-you're not a spacerocker
. Your tapping skill is OK. So, you tap in the beat of the drummer. Although you're a bit off, the time is not noticeable at all because there's only 3 repeats. Plus, the echo is not set by a constant rate, it echoes whenever you play (more on this later). So, it all sounds dandy.
Now, let's consider tap tempo modulation effects. The tap tempo sets the speed of the internal LFO, which is then constant no matter how you play the guitar. I'll use the ModFX ampliton as my example, just because the ModFX have some pretty standard tap tempo features. Back to the scenario.
You're playing with a drummer who keeps the beat OK. You want to add some light tremolo for your lead part, to make it all pretty sounding. So, you turn on your very digital Ampliton to triangle wave form. You want the tremolo to be in good beat with the drummer, just because you want it all to fit together. So, you tap in the beat as best you can. You're OK as a tap tempo-ist, and as you play, it sounds OK. However, since you were 20ms off, every time the LFO completes a cycle you're another 20ms off. The LFO is about 1 cycle per second. After maybe 30 seconds or so, you're a very noticeable 600ms off beat, and the sound sort of gets off...
See? With delay, the repeats are only "spawned" when you play-so as long as you're playing in beat, your repeats will be reasonably in time. If you had infinite repeats, they would get off time eventually, because you're not the greatest tap tempoist and your drummer isn't always on beat. But, since you only have to worry about 3-10 repeats or so, the off-ness isn't noticeably, hardly at all. But, with LFO based effects, at each waveform the off-beatness gets successively more and more off. Eventually, you're totally off. You and your drummer have to be perfectly in time for this to work, if you want your tremolo or phase or filter or whatever to be "in time". So, let's go back to the scenario one more time, with a non-tap tempo tremolo effect.
Your playing with a drummer who's not all to great at keeping time, but hey, you're desperate for a drummer, so you play with him. You want to add some light tremolo to your effect to make your rythm parts sound a little more interesting. So, you flip on your handy dandy analog VooDoo Labs tremolo to a reasonably high speed that has nothing to do with any part. There's no reason this sounds bad, I mean, it's just tremolo. You don't need to be in time. It's just to add a light effect to your sound, not to be perfectly in time. So, you go on and play with your OK drummer, happily ever after.
See? That works just fine. If you consider the same scenario (with a non-tap tempo) delay, and you have shorter delay times, it'll be fine. But personally, once delay times get longer, I find it very very useful and very together sounding when the tempo is on with your drummer/other band members/loops/etc.
I hope this makes sense to you all. I base my opinion on the fact that I now couldn't live without my DL4's tap tempo, and that I never use my ModFX philtre's tap tempo because it will eventually get off (sometimes sooner than others).