Do you think that baseball fans in general (not like, people who go to games but people on this site for example) struggle with seeing grades for a guy as opposed to just an overview of what they look like and comp to? I generally find grades to be better because it's easier to compare, but it obviously leaves something to be desired.
Random Pet Peeve of mine - Kiley McDaniel wrote something for Fangraphs saying Buxton has "90" speed because he's literally off the charts scouts use for speed (3.9 to first base). Drives me nuts. Just say he has 80 speed. Not all 80s are the same, just like two guys who are 50 for speed won't be the exact same.
I think the challenge writers have to deal with in writing about player evaluation is deciding who their audience is. If you have industry folks in mind, you need to speak in their tongue (which means grades/terminology). If you want a braoder appeal, you need to be able to convey what those grades/terms mean to the average fan. In other words, if you are responding to a post by Chris, or sports guy, or rznj, or all-star, or can of corn, or tucker, or INSERT POSTER NAME HERE, are you comfortable that they all know what a 60 arm, or a "plus" arm, from third base looks like? Is it more useful to your audience to say "Machado has a plus to plus-plus arm at third," or "Machado has plenty of arm strength to make all the throws at third"?
At the end of the day, it's all about conveying info to your audience, whether it's writing for a blog or message board, or submitting a scouting report to a supervisor.
Re: "90" on the scale, in my opinion, I think it ties into the above. Stylistically, writers have their own way of conveying info to their audience. There's an area scout in my region that has more than once chuckled and said, "that's a ten" or "that's a one" when referring to a really fast/slow home-to-first or 60 time. It's probably not what I would do, but at that point we're really parsing, right? What's important is that the info is conveyed. That said, if you are annoying your audience with turns of phrase, that's probably not a good thing.