I think the idea of Termiantor armour is two fold.
1) In bording or siege actions (including trench warfare) where space is very limited and your facing pretty standard foe at close range. One Terminator (let alone 5) will simply be able to dominate a horde of rebel guardsmen in such circumstances for example who can't bring their superior numebrs to bare and would get trampled underfoot by the Tactical Drednought (in theory).
2) In actions where your facing monsters, aliens etc who are fast and/or strong enough that a dodge or parry won't be enough to prevent the marines from getting hit. Genestealers are the classic example. They have 3 or 4 attacks each. A marine in power armour may be able to dodge or parry one or two hits with the right skills but then will still be facing two or three more hits. Even if he survives he then has to kill the xenos himself.
Space Hulk Cleansing Doctrine dictates that the Astartes are better off loading their teams with more powerful weapons (powerfists, storm bolters assault cannons etc) and heavier armour (and a field save) to have a decent chance of surviving the assault and then killing the xenos critter in return,
Make no mistake even in Termiantor armour Space Hulk and boarding actions are considered incredibly dangerous but I guess the Astartes are playing the percentages.
In situations where power armoured marines have numbers and the space to move and use cover then of course they are going to deploy a combined arms approach of a few marine squads, maybe a tank of some kind.
Finally the Imperium favours intimidation as a form of warfare. Regardless of their effectiveness against well armed and trained troops, against a foe that has never encountered them before a squad of Terminators teleporting into a Command HQ may have the ability to end a war! (although I suppose the same could be said of other marine squads as well).
I have a question/comment though. Previously it was always stated that it was only Terminators who had the ability to teleport because their armour insulated them from the worst effects of the Warp and also had the bulk to carry such gear as personal teleport homers for extraction. However I am sure I have read in more recent Black Library stuff about power armoured marines teleporting. Certainly in Rogue Trader (FFG) the ship can have a teleportium that gives no restrictions on who can use it.
Just wondering what peoples thoughts were on this? The unique ability to teleport may make a difference as to the effectivness of the armour.