Welcome to the Forum
Lasers are a fantastic training tool. You get impressive visual feedback of how steady you are and how smoothly you pull the trigger.
They can rapidly improve a novice's shooting ability as he/she learns what works and does not work. I have lasers on roughly half of my teaching firearms just for this reason
They also help the more advanced shooters to quickly hone his/her point shooting skills.
While not a huge concern, they will allow much more accurate shooting from cover (barricades, corners) during those times that you can not look straight down the firearm
Should you rely 100% on them? Of course not. You should not rely 100% on anything. Man made products fail. Front sights shoot off, magazines drop their cartridges, night sights lose their tritium vials, cylinder hands break, etc.
Never wait for the batteries to go dead, change the batteries on your birthday and you will probably be in good shape throughout the year. Kind of like how we take care of our smoke detectors batteries
While a Glock guide rod might or might not fit an SD, the laser guide rod probably won't work. The on/off switch is usually in the takedown levers.
You can see the on button located in the lever on the lower firearm. It can be activated from either side. Not sure where the switch is on the SD version
Just because they no longer make the guide rod laser for your firearm does not mean you can not buy one. They often turn up used on the various forums and auctions sites. You could even post a WTB right here on SmithWesson-Forum.Com