The valve adds an extra fourth of tubing to the instrument - changing the fundamental pitch of first position to F, rather than Bb. However, I've been using the valve as either an articulation device, filtering mechanism or percussive device to pretty good success.
Air sounds through the valve, both hard and soft cuts
Basically, when I change the length of the tubing very fast, the momentary change in air pressure creates a suction which results in a hard articulation. I can put soft or loud air sounds into the instrument and use the valve to kind of chop this up. I can also fluidly move the valve to transition between these states - while the slide is placed normally, this doesn't really change much (except when blowing at the mouthpiece to achieve pitch, then you do get a pitch change), but with the valve removed this slow action can get really interesting effects between the bell and this "secondary bell". like I can use it to crossfade between the two in an interesting way. This same action can be used with ordinario playing, which acts as a crude low-pass filter, creating a more sinusoidal tone.
Removing the valve's tuning slide
This action is really effective - Eric Wubbels's new duo uses this setup as the default setup for the whole piece. Basically, I remove 1/2 of the valve's tuning slide and so every time I depress the valve, the sound is routed out to this other bell. Since it does not have a bell flare, the acoustics of the tube are totally different. In the folder of scores, Mike Svoboda's concert etudes show the new overtone series created by this. On it's own, this new timbre isn't the most compelling, but when used with split tones or air sounds or other attachments such as a bassoon reed, it's really compelling. I can effectively do split tones "out the back" of the horn through this open valve (see the beginning of the 4th movement of contraposition - the only thing I'm doing is just releasing the valve).
Gradual transitions on the valve
Using the valve as a filter is *really* effectively with granular sounds. Either with the slide removed or used normally, the indeterminate states between the open and depressed states of the valve are extremely interesting. I'll have some audio below playing this transition, in both configurations. The half valve one is super nice, it sounds like the sound is "submerged"...