Getting back to topic. Some time ago my friend played his 1000 Blood Angels against footslogging Necrons. His assault squads were bogged, his Libby was slain by Necron Lord, his Devs were shot down... His tact was unnoticed. He won the game because the tact remained on objective, and his opponent didn't see tact as a threat.
This has more to do with very poor strategy by the opponent than with the cost effectiveness of tac squads. I've had the same thing happen with scouts. They were ignored and then able to run to an objective and win the game on what turned out to be the last turn. I'd never recommend taking them on a regular basis, over other choices simply because my opponent made a mistake. You can bet he wouldn't make that mistake a second time.
The thing is that many people don't take BA tacts seriously. A few bandwagon jumpers in my LGS keep saying "this vanilla piece of ! We don't need it". But they prove to be wrong. Tacts are very useful, as they don't have to be at the front line to be extremely useful.
Implying that tac squads are "necessary" is a bit of an exaggeration. As I've said, I think they're fine. Tactical squads are really only good at one thing: Their combination of some long range firepower AND being a scoring unit. We have units that can do both more cheaply and effectively, but not necessarily at the same time.
In fact, as I was thinking about this, I wondered if the following might be a better idea for a long range firepower "unit" that can hold an objective. 5 Devs + either 5 Tacs or Scouts. Add in as many ML
as you want with the Devs and, if you take them, the scouts can actually take a ML
themselves. An improved firebase over the 10 Tacticals for roughly the same points. Once again, I'm not saying tacticals can't have a place, but, as mentioned, I haven't been taking them recently and I certainly haven't missed them.