While i do tend to agree that Guardians make for an excelent defencive unit (big, meaty squads of them are the best thing we have to hold Objectives now, IMO) i feel i should point out that your tactic, Vampa, wouldn't work.
Unless said Bastion is dilapitated, or you manage to swindle a good deployment zone from your opponent, you can't have both a Bastion and an Aegis line. This is because, as per the new Force Org chart, you only have one Fortification slot. If you were to take the Aegis line, you couldn't manually shoot weapons in the Dilapitated Bastion, and if you took the bastion, there are unlikely to be any guns on the Aegis line (which any sensible players would just dub empty barricates).
Otherwise, the idea is solid. If there is something the Eldar are good it, its lots of shots, and with Overwatch charging a heavily fortified possition of Eldar is a daunting idea. I wouldn't say it's fluffy, but it is certianly a viable tactic again.