Exchange units in 40K

How do you feel about them?
There is a brand of unit in Warhammer that I term the Exchange unit.  It is a unit whose characteristics make it ideal for killing certain targets but at the same time are quite likely to die in response to their effort since they are most often taken at minimal unit sizes. 

An example is 5 Swooping Hawks.  They are good at killing vehicles, but are also likely to be vaporized on the ensuing round, as an enemy can scarcely allow them to go bouncing about his parking lot a second time.  Their weak stats and decent but surmountable armor will insure that moderate effort will end the small unit of Hawks.

As a matter of strategic philosophy (in other words the overall plan going in moreso than the logistics of carrying out the plan) I know that First Blood makes an exchange unit a more risky proposition.  I also observe that Kill Points do matter in numerous missions.  Exchange units are by definition sure to die.  In tourney play that can make it difficult to win if you over invest in such MSU tactics.

Well first, strategically, the best place in the list for exchange units is in the Elites slot and the HQ slot (assuming you have 2 HQ's to avoid losing your Warlord).  If you take them from the Heavy or Fast Attack slots, that could penalize you in the Big Guns Never Tire mission and the Scouring mission.  Best to use Elites, as a practical matter.

So right off the bat, on the macro level of army and battle planning, we know where thse units should best come from but is there an exchange unit worth taking in Elites/HQ in your codex?

Many codex's have Elite/HQ units that can be used this way.  Eldar have Fire Dragons which may be one of the most well known exchange units.  Sisters of Battle have Celestian squads which really function well as Fearless Roadblocks to tie up the enemy until the Sisters next turn; so not an offensive juggernaut, but an exchange unit with a specific purpose.  Imperial Guard Storm Troopers could be considered because they can carry a fair amount of special weaponry.  Tau are famous for their "Suicide Command suit".  All of these fit the bill.

What all of them have in common is, you're conceding them to the enemy.

My question is, as you look at your battle plan, do you prize offense so much that you would abandon defense this way?  Keep in mind that just one member of a unit left alive denies the kill piont and allows you to score/deny with it.  Keeping unit fragments alive and fading them back can be an important part of your strategy.  If the unit is too small, then it is just a 1:1 exchange at best and has only weakened you if the enemy has more units than you do.

Overall I do not like this exchange idea for a couple reasons:  The 5 Fire Dragons will kill the LandRaider and die (for example), but if it had 3-5 ablative wounds in the unit, you're going to kill two LandRaiders with the same Kill point and/OR potentially live to see the end of the game! Since the enemy's firepower becomes less over time, the unit fragment will become progressively less endangered.  Yet it still adds to the target saturation issue when it would not have before!  So the ability to press forward after terrific losses to a unit is key.

My suggestion here in the end is that if you are sure you must have the three 5-man squads of Fire Dragons or the Suicide suits, reconsider and add bulk to the units.  That means another part of the list "suffers" (an overused term if ever there was one) but it also means you're not just throwing away the KP's that can be a big determiner of success.  Since exchange units are in the Elites/HQ slot, the enemy isn't firing at the more important slots in the army so in some ways the extra wounds are really wounds that you could consider to have BEEN in those other units!  Think about it:  if they weren't in that exchange unit, the enemy would have fired into the more important unit without fear of the now dead Elite.  Abstractly, do you really care how many die in total?  Not as much as you care which ONES died.  So by forcing the enemy to pour more of its units into THIS task, you are giving the scoring components of the force more shelf life.

Food for thought.