About gear blocks, it seems a lot of ARFs are designed with the assumption that you'll be flying off of a paved runway, and that you always grease your landings perfectly. But when you fly off of a rough grass field, it's not uncommon for the gear mounts to take more abuse on take-off runs than they ever would on pavement. I've seen several different kits that suffered from this.

On firewalls, first, there is the issue of glue joints not being right from the factory. Better safe than sorry here, and it's not that much weight. Second, on trikes, the nosewheel mounts to the firewall, and all the above about landing gear applies double to the firewall now. Lots and lots of trike trainers loose their firewalls from the kanagroo "landings" the plane encounters. Third, a lot of guys don't bother to balance their props, spinners, etc. So the firewall is subjected to a LOT of vibration, and that eventually works the firewall loose. A bit of extra strength means a longer airframe lifespan. (for comparison, get a look at a well designed electric planes front end compared to a glow plane. Electrics have much much lighter construction in the firewall area just because of the vibration. Fourth, guys have a tendancy to put oversized engines in our planes. When you do that, the #3 becomes even more of an issue.

As for tail surfaces, reinforcement helps keep things attached when the over-sized engine from #3 above winds up. It also helps with hangar rash.

Things like wing holddowns are frequently reinforced because guys frequently put those kinds of planes though higher G's than they were designed for. I cringe when I see guys at the field doing full power loops (as in, full power all the way around) and such like that. Many of these planes just aren't built for that kind of thing.

and lastly, sometimes a design really does have a weak point. Something that always fails if you fly past a certain G loading, but is easy to fix. Sure, something else will give instead, but it might fail quite a bit later, and give you that much more freedom to not worry about it.