Reasoning behind DST/summertime arrangements ("STA") is either harmonization or obsolete benefits from pre-lightbulb-era. If you have a good proven reason which isn't solvable by sticking to either standard time or daylight saving time, please leave a comment.
For ditching STA here's my engineerish reasoning:
It kills people
Imposing jetlag for the whole nation by law is silly. A few studies with nice keywords for further searches:
- The Human Circadian Clock's Seasonal Adjustment Is Disrupted by Daylight Saving Time
- Daylight Savings Time and Traffic Accidents - NEJM.org
- In 2008 - Daylight Saving Time: Clock-shifts Affect Risk Of Heart Attack - ScienceDaily
It's nuisance to adjust clocks
We still have a lot of timekeepers which are not aware of timezones nor have any means to synchronize time from an external source e.g. traditional wrist watches, wall clocks, grandfather clocks and clocks on typical home appliances like microwaves and ovens. In the worst case you have to find the manual to get the job done.
Whether you loose or gain extra an hour doesn't matter as by end of the year you have only wasted time adjusting to it twice.
It's messes up daily statistics
Let's assume your input is a result of people's daily activities. A graph depicting a fairly quiet night, it gathers speed during morning as people wake up and buzzes steadily till midnight. If you're analyzing this data in your local timezone you have to deal with 23 and 25 hour days. Or in case you work with UTC timestamps, the normal cycle will be shifted -+ 1 hour.
It results in wasted opportunity and work
As a human goes around adjusting clocks he or she might miss a few knobs. Clock running an hour late or early might cause an alarm system go off, locks engaging or disengaging at wrong time, blinky yellows at a busy intersection, people going to meeting/work an hour early/late, heavy machinery working on empty conveyor belt and so on.
A friend working in security industry said the day after adjustment is very busy. Distinguishing between a real alarm, one caused by software misconfiguration or person with a skewed clock might get tricky when you already have your hands full.
Software development & integration
Nowadays, developing software with latest and greatest platforms and frameworks, our friend timezone database helps keeping things in order. However the other end of you integration, a legacy system from 90's, might be totally unaware of STA or running an outdated version of the timezone database as further updates are no longer available. Just a few things I've bumped into
- Mail servers shutting itself down because time had moved backward
- Database timestamps one hour off
- Software miscalculating time intervals across DST and timezone boundaries
- Discontinuous logs (3:00 3:15, 3:45, DST OUT!, 3:00 again, 3:15 ...)
As the adjustment is a rare occurance it might get overlooked in testing and a nasty bug might slip through the cracks.
It clutters user interfaces
Whether it's on a paper, a punch card or a device and there's no room for error, there has to be field for timezone (+0300/+0200, EEST/EET for finland). Let's say a medical device displays elapsed, start and end time, and dosage. The device gradually ramps up the dosage during an interval and you need to verify it's operating normally. Without timezone indication it requires a bit more brain flexing for verification during STA adjustments.
Other than slaying people with heart attacks and in traffic accidents all these problems can be solved. The core issue is the man-made minefield of problems associated to STA and we have to solve them without any benefit.