Ok, I've been thinking about this thread, and figure we've got explaining to do. In general, I agree that modulation effects don't usually have wet/dry mix knobs (please enlighten me on the ones that do). This is because the way you build a modulation effect is to mix your dry signal with some modulated, delayed, and/or filtered signal. Other times the mix knob just doesn't make sense, and wouldn't have an outcome any different than turning the depth to 0 (filtering effects, tremolo, vibrato). That being said, the intensity knob the MF algorithms does already actually do wet/dry mixing in the majority of cases (thought not always to completely dry). I'll go through this per algorithm.
Chorus: Among other things, the Intensity knob does control the wet/dry mix of the original signal and the delayed / modulated voices. Doesn't go all the way to zero, we assumed people would just bypass.
Phaser: This is really a filtering effect, achieved my modulating allpass filters (filters that effect phase or small values of frequency dependent delays, but pass amplitude at 0 dB), so the explanation below for QWah kind of applies. That said, it the Intensity knob still does some wet/dry mixing, though, like chorus, not all the way to zero.
Q-Wah: doesn't make sense to have Wet/Dry. Adding the dry signal would change the filtering response to something more like a peaking filter.
Flanger: Same as chorus
ModFilter: same as Q-Wah
Rotary: This does have a wet/dry, even though we didn't think it made too much sense to do it at first, but we liked the sound of it (comb filtering and all), so we implemented it after people asked for it.
TremoloPan: Doesn't need a wet/dry, use the depth for this or simply bypass it.
Vibrato: Intensity is wet/dry mix
Undulator: Intensity is wet/dry mix
RingMod: Intensity is wet/dry mix
So, in reality, we're only actually talking about three algorithms, of the which the whole effect is accomplished by mixing your dry signal with other signals, and the Intensity knob on these is already doing wet/dry mixing in order to give varying degrees of the effect.