@Rimas,
While I agree with what you are saying I only agree to an extent :P I have a dual Xeon that reaches 3.8ghz using a single core and its slower in lightly threaded tasks than my i7 4790k @ 4.4ghz. From my experience the difference is not that huge. Especially not so in the viewport. Some sims take longer to run but generally for most stuff I don't notice the difference.
That said, in IPR and any multi-threaded tasks (rendering, video conversion) the difference is obviously huge.
So from my point of view its like taking a little out of the single threaded speed and gigantically improve multi threaded speed.
Ultimately I do think it depends on what you do. If you mainly do smaller / medium sized scenes the difference I think its negligable compared to the rendering boost (IPR).
If you do VFX and mainly render / sim on 2-4 threads, ha, its probably better to go the other route and invest into a 4.8 or whatever CPU :) As a matter of fact, the 4790k I used to use before the Xeons is now the perfect workstation for my significant other. She is using it in Marvelous Designer and the speed gains on those 2-4 threads are really big for her.
Being primarily a c4d user myself I can also say that I haven't noticed a big difference in viewport speed in the said app. C4D traditionally really likes fast cores.
Even if the viewport has 20fps instead of 30fps (fictional numbers, haven't done the tests) I can still get the previews so much faster than before because of the IPR speed... Obviously something like MD would be a different story probably.
Now like you said, and I totally agree with you on this part, if the max speed for your CPU is like 3.2ghz I think you'll definitely notice it being slower in quite a few areas. Heck, the speeds you posted a few posts back are lower than my current all-core boost.
The biggest drawback is obviously that to get above 3.4 ghz on a dual Xeon system you probably need to pay through the nose for it.
I mean thats just my point of view anyway. Like I said, I think it depends on what you are doing. From that point of view GPU rendering seems fun because you can load it on a highly clocked / low core CPU and still get great rendering speeds. There are obviously other limitations there though...
I was very skeptical of the single threaded speed before buying the Xeons but in the end it turned out to be a great choice for what -I DO-. :)