One thing thing could be good for, is if you just enjoy the questing in WoW and don't like to be bothered by a lot of other people. You can take your time and level up to 20 as slowly as you like, and the low level zones don't have a lot of people in them. Even once you cap out at 20, you can go and do almost any of the zones with quests that don't require a level higher than 20. Or you could just reroll new characters for each of the starting zones.
Blood elves and draenei seem to be available to trials, so the only starting areas that are off limits are goblin/worgen/death knight.
One of the advertised limitations is the inability to join a group with someone over level 20, but I invited one of my trial characters to a party with my level 85 character. Maybe this has to do wtih them both being on the same Battle.net account?
RealID functionality isn't actually disabled like they say. It's more "hidden". I created my trial account as a new account on the same Battle.net ID as my main account. People who have me on their RealID friends list see when I log onto my trial account, but I only see people in my friends list if I add their character. I see no popups when people log on/off, but they see them when I log on/off.