Speaker Compression is legitimately a more vintage sound. Early Bluesbreaker 18watt sounds require speaker compression. Early Cream tones benefit from it also. It even reaches out toward early ZZ Top. Lots of those greasy swampy blues are built around it. I think it is a great tone. I only wish I didn't have to sacrifice dynamics, punchiness and pick-attack/onset-transients for it. But, sometimes it is worth it. And sometimes it fits. Speaker compression is definitely part of the tone palette.
I have to agree with the following:
I think many of the more old school guitar sounds are impossible to re-create without it.
I think, it is an important part of some sounds. Celestion Blues in an AC30 on full tilt compress quite a lot. Also the small old Fender amps with AlNiCo speakers can be fantastic when they are driven to large amounts of speaker compression.
However, Speaker compression is not a major feature for most modern sounds. Especially, when "the spirit of the age" wants its crunch extra-crunchy. And, post-SRV blues has brought more emphasis on tight picking dynamics. Speaker compression is not as prominent in newer speakers or rigs. And, most modern players want a tighter, more modern, sound. Speaker Compression is just not that in line with current sounds.
Also, as amp models keep becoming more and more organic and complex, speaker compression is no longer being used as a supplement to an amp model's complexity.
I still use Speaker Compression.
I like tighter sounds; High-Voltage, low-to-zero PI Bias Excursion, Hot Bias, Hard Power Tubes and Clean OT's.
But, I still use Speaker Compression for some of my more unique and more vintage preset sounds.