I'm using a PIC12 in product that replaces an existing one. At Volume Pricing 8 Bit PICs are pretty much incomparable to ARMs. A quick check on Digikey shows that for 10000 pieces, the cheapest 8 Bit Micro is a PIC10 - 35Cents/Unit (that's for today only). ATTiny is about 2 Cents more but that can change. The cheapest 8051 from Energy micro is in the same range as well.

The cheapest ARM Cortex M3/M0 or even a ARM7 is $1+. When you are shipping products (and not doing a hobby project), BOM cost is a major factor. If you sell a Million Units each with a micro in them, the cost difference is about $700,000. Even keeping cost aside, size matters. The smallest 8 Bit micros from Atmel or Microchip or any 8051 is around 3x3mm. Most ARM micros are much larger.

Rule of thumb that I use is to pick the micro by function, price and size. Sometimes a 8-Bit micro is more than enough for the job. Buying something that costs more, bigger in size and for which the MHz is not an advantage is an overkill.

Having said that, a companion product that also working on requires the MHz and so, a Cortex-M3 fits perfects there.