Sleeves for manuals definitely recommended, considering packaging wasn't that great back in the day easy to damage
but for ps2 era and up it's overkill and not needed. ps1 dreamcast are the final era that would need them. unless your living in the us where ps1 and dreamcast manuals are your cover.
Plastic cases for dvd cases are overkill imo, same for plastic cases such as ps1 dreamcast etc unless they are sealed. just makes it harder to use them, plus when stored normally it ain't going to get scuffed. if they are factory sealed than sure but otherwise it's not worth imo, doesn't look great either for these dvd/plastic case games.
As for the manuals issue, trust me I'm right there with you on that. I've started bagging up my manuals for my CIB N64/SNES games. The N64 manuals are especially prone to damage every time you try and take the game out, just because of the way the inner trays are designed inside some N64 boxes. I should probably make a video on that one day to show how I store the books once all bagged up.
even if the trays where excellent N64 snes, nes manuals are brittle. this ain't anything like ps2 gamecube and modern era manuals. These ain't sturdy they'll wear down over time.
Games looks just fine to me in the protective plastic cases.
The plastic on the DVD's and PS1 are fragile anyway, and I prefer it this way.
But each to their own 
In terms of looks to each their own indeed.
ps1 cases are fragile but a plastic case doesn't really protect against that I'd say. if it falls they break, wether they are in a plastic case or not or if they break by using them. potential shelfware is the only protection you'll get from them but in practice when you store your games nicely with plenty of space that they aren't squished together it's going to be minisule in terms of shelfware if any. this ain't cardboard in which almost anything will damage it. dvd and ps1 cases are pretty durable in that deparment, moreover if the worst is to come quite easy to replace unlike cardboard. the art work is the only thing that really matters.
My clean sleeves still look real shiny because it's stored decently plus the fact that most ps1 games will have some shelfware on the cases when you get them. and that's already nice condition when they don't have any cracks etc.
imo it's only potentially usefull on games that came right out of the seal or the difference won't really be noticable. unless you really handle your games poorly. and even with those when stored properly it will pretty much remain identical. for ps2 you could find some mint sleeves, but for ps1 that's going to be allot harder. heck if there ain't any cracks and it doesn't have allot of shelfwear your already at the top end of ps1 plastic cases. and anything below mint is going to have miniscule differences in terms of shelfwear protection.
keeps the dust away so that's a plus I'll give ye that one. my 2 cents on plastic sleeves on dvd era or ps1 cases.
plastic cases are nice on dvd era cases when you sell stuff at a boot or something, when allot of people handle them games you can get some ugly results even if all customers would handle it decently, but in terms of home usage I wouldn't worry about that.
