about a month ago, a 175w merc went out on the other side of town, and a lineman came and put a 100W NEMA on to replace it.  Ok, fine.  I went just today, and he has since replaced it with a 400W HPS Cobrahead?  Does this make any sense to anyone?

Someone may have requested to have brighter lighting for that area? 

Brighter? Isn't a 400watt HPS lamp about the same as a 1000watt DX MV? Not logical whatever the explaination.

Sadly, I find this to be very stereotypical of real world thinking for the past 50 years (present board excepted).

Municipal lighting service managers seem to know very little on the various technologies, and lumens per watt. Where I live the 400w DX mercury is still the workhorse of 90% of the town's streets. When relamping doesnt fix a failed fixture (ballast finally went after 60 years), they install an HPS cobrahead of the SAME WATTAGE. Now, the brand new shiny 400w HPS cobrahead is kicking out nearly 4x the light than the old mercury cobrahead did (nearly all of the 400w mercury cobraheads around here have extremely old lamps that that produce anywhere from 1000 to about 8,000 lumens at best, the drop lenses are also filled with bugs further decreasing light output). The few people I know that live on those parts of a the streets where the blinding 400w HPS went in complained to the city, and they couldnt careless.

Yep big waste. They don't even care that they could have saved tons of money by selecting lamps with appropriate lumen output. Shouldn't it our jobs to help them out, save these towns some money and save the planet at the same time?

In my home town, when they replaced the MV street lights with HPS, they tended to go for near watt-for-watt equivalents, meaning 175 watt MV NEMAs got replaced with 150 watt HPS cobraheads, and 400 watt MV clamshells and cobraheads got replaced with often 400 watt HPS cobraheads.  One particular main street that was mostly lit with Form 400s, with only occasional HPS cobraheads, got relighted with what seemed to be whatever was on hand at the time.  The new lights are a mix from 200 to 400 watts, and a mix of AE, GE and Line Materials lights.  The lights on this street are spaced close together and low, so 200 watts each should have been enough.
they recently went through and replaced all the residential streetlights with GE LEDs, and they seem to be a lot more uniform.  Major roads use either various LEDs, HPS, or some now use MH.

Icefoglights, was this Fairbanks you're talking about?  It would've been neat to see that all lit with mercury back in the day.  Do you know of any mercury cobrahead fixtures there still in use on side streets, etc?

None what so ever.  By 1994, all municipal street lights were HPS, and many of those are LED now.  The last pocket of MV was a neighborhood who's road service area also owned the street lights, but those were all replaced by 2007.  There was one MV street light put up since than, I think by a private developer, but that has since been replaced with LED as well.