Impressive, Patrick. I could say that I’m concentrating on the music now, and don’t need any more whistles, but, when I arise on the morrow, who knows…what evil (spelled w-h-o-a) lurks in the heart of man…only the whistleman…whhaaaaaaaahahahahaaaaaaaaaaa! 
Philo
Well, there’s the answer, then. Now that I’ve achieved nigh perfection as a player, I just don’t need all those whistles anymore. 
Actually, for me it might be just the opposite. Concentrating on one whistle seems to make me a better player, especially when it comes to “personality” issues such as breath vibrato (which each whistle responds to differently), notes that need blown into tune, etc.
t
I’m with you, Badger. I very rarely play anything other than my oak D.
J.
For me, I’ve found that playing different whistle makes (and in different keys) has actually helped my playing quite a bit.
There was a time when I, too, thought I should focus on only one or two whistles. When I started branching out, I found that – strangely (or so it seemed at the time) – my playing on the whistles I’d been concentrating was getting much better.
Perhaps it’s because I need to be flexible when playing substantially different whistles – they require different fingerings, blowing, positioning, etc.
Moreover, I found this was also the case when I started learning the Irish flute about a year ago – my struggles with the flute made me a better whistle player.
Of course, others have had exactly the opposite experience.
Walt
Yeah, especially when WORLD FAMOUS flute makers keep sending you their best stuff!!! 
I’m happy with the collection I have…“collection” being of a streatch. I have 1 cheap D whistle (I don’t know the brand), 1 Clarke Origional in D, one D Alba, 1 low D Alba, 1 C flute (inexpensive), and 1 G flute (also inexpensive). At the moment, I have no desire to acquire more, because I prefer to not switch back and forth constnatly and because I’m cheap.
My WhOA and FlOA are actually sublimations of my deep-seated compulsion to give noogies to people in authority.
Last night I had several dreams where I was searching for someting–the last dream I was searching in the water at a beach.
Yes, I had spent part of the day been looking online at new whistles!
…perhaps a sign that I should STOP??
But seriously, I don’t really think WHOA is a constuctive sickness. Whistles do play differently, and when I’m playing in my basement switching whistles for different sounds, I frequently overblow my Syn, underblow my Gen tweaked Bb, and basically screw up on my Burke D.
I’m a newbie in the whistle playing, as I began during last summer.
I don’t suffer of WHOA, but I’m infected with another desease, the WHOB : Whistle Obsessive Building disorder ! 
I’ve already built 2 PVC D, 1 PVC/copper D, 1 copper D, 3 aluminium D and 1 PVC/copper Bb.
The last whistle I built is always my prefered one : I play it during a few days, indentifies the pros and cons and begin to build another one trying to keep the pros and suppress the cons. Well, the results are not always the ones I was thinking of, but I learn a lot about whistle design, and it’s really funny !
Tagea, sounds like we might see your name on a Google custom maker’s list someday!
I prefer a delusional approach to this multiple-whistle quandary. I simply pretend four whistles are one. And how, pray tell, does this play out?
Well, for instance, I’ll take my four Low Ds, and play the same tune with each of them consecutively. The Copeland, the Overton, the Burke Viper, and Burke composite. Each whistle makes the tune different. Mood…color…feeling…approach…conclusions. Each rendition informs future renditions. Cross fertilization. Cross reference. Pure fun!
If I find another Low D that’s up to par with these, I’ll gladly add it to the party, and STILL HAVE JUST ONE LOW D! Do you see how beautifully this method of WHOA-control works??!!!
It’s a tough life, but someone has to live it.
Doc