I admit it: I'm a grammar nerd. I know the difference between 'who' and 'whom', and I'm proud.
I used to be pretty militant, but these days I'm more relaxed. I still take joy in the mechanics of the language, but I also believe that English is defined by its usage, not by a set of arbitrary rules. I'm just as happy to abuse it as to use it, although I still think it's important to know what rules you're breaking and why.
My approach now boils down to this: language is like clothing. You (probably) wouldn't show up to a job interview in your pyjamas2, but neither are you going to wear a tuxedo or ballgown to the pub.
Getting commas and semicolons in the right place is like getting your shirt buttons done up right. Getting it wrong doesn't mean you're an idiot. Everyone will know what you meant. It will affect how you're perceived, though, and that will affect how your message is perceived.
And there are former rules1 that some still enforce that are nonetheless dropping out of regular usage. There was a time when everyone in an office job wore formal clothing. Then it became acceptable just to have a blouse, or a shirt and tie. Then the tie became optional and now there are many professions where perfectly well-respected and competent people are expected to show up wearing nothing smarter than jeans and a t-shirt.
One such rule IMHO is that 'data' is a plural and should take pronouns like 'they' and 'these'. The origin of the word 'data' is in the Latin plural of 'datum', and that idea has clung on for a considerable period. But we don't speak Latin and the English language continues to evolve: 'agenda' also began life as a Latin plural, but we don't use the word 'agendum' any more. It's common everyday usage to refer to data with singular pronouns like 'it' and 'this', and it's very rare to see someone referring to a single datum (as opposed to 'data point' or something).
If you want to get technical, I tend to think of data as a mass noun, like 'water' or 'information'. It's uncountable: talking about 'a water' or 'an information' doesn't make much sense, but it uses singular pronouns, as in 'this information'. If you're interested, the Oxford English Dictionary also takes this position, while Chambers leaves the choice of singular or plural noun up to you.
There is absolutely nothing wrong, in my book, with referring to data in the plural as many people still do. But it's no longer a rule and for me it's weakened further from guideline to preference.
It's like wearing a bow-tie to work. There's nothing wrong with it and some people really make it work, but it's increasingly outdated and even a little eccentric.