I have been pondering this for a while and would like to know how other people feel about the inclusion of the DeathWatch into the 40K universe.
I recently strained to come to terms with the Ultramarines allowing their brethren to serve in the DeathWatch and the whole ethos of Tyranic War Veterans, I can see why GW has done it from a marketing point of view but from a personal fluff view I think the force makes little sense.
Essentially you have a chapter packaging off some of its most promising young warriors to fight in a multi-chapter fighting force, gaining experience and defeating xenos along the way. However this because extremely strained when you include their command as the Inquisition. What we have now is Inquisitors commanding elements of the Astartes, a thing no SM Commander really wants as they enjoy their independence, and also very few chapters trust or listen to the Inquisition. The involvement of the Inquisition in the Badab wars led to the Lamentors joining the Astral Claws, thinking the Inquisition was declaring war on the SM Chapters.
Furthermore when you look at the low level aspects of the DeathWatch you can see several fatal flaws in sending off warriors to a multi-chapter force:
1. The apothecaries, if any will most likely not belong to the parent chapter of the fallen marines. This is a major problem as an Ultramarine removing a Blood Angel geneseed is not going to follow all the correct rituals, etc... and so the geneseed is essentially lost to the chapter.
2. All Chapters have their own methods of war, so by sending a marine from the Space Wolves to fight alongside the Salamander you achieve very little, the SW will want to charge the xenos and hack them to pieces with Blood Claw like glee while the Salamander will want to decimate them with hot lead. These various modus operandi cannot be supported in one squad, which is how the DeathWatch portrays them and so brethren must adjust their methods. Which again hampers their effectiveness when deployed back with their parent chapter.
3. When the brother marine returns to his chapter he may have been away for some time, possibly just guarding a necron tombworld and so will have little knowledge of his parent chapter's recent history and few ties to his brother marines, something terrible for units like Space Wolves who stick together their whole lives in packs.
4. The Chaplains of the parent chapters will not be there to help prepare their brethren for battle or fortify them against gene flaws such as the Red Thirst or Curse of Wulfen, meaning more problems.
5. One of the most effective motivators in the SM is devotion to the Emperor and the Chapter, if this devotion is replaced by devotion to their current comrades then the DeathWatch is essentially sowing seeds whereby loyalty to the Inquisition (Ordo Xenos) will supercede the parent Chapter, something to think about.
6. Several chapters hate each other and so you couldn't have them in the same battlefield.
7. If the parent chapter is based in the Wheel of Fire fighting Orks then what point is there for the Chapter Master to second his men to help fight a Hive Fleet? He gains nothing as the warrior will probably return with knowledge they never use or could get from other chapters (they share information you know).
8. If you send the marine off to war can you replace him? He may return and suddenly you end up with an additional 300 marines, something bound to turn heads in the Inquisition or quite useful if you want to rebel, send off 400 men to the Ordo Xenos for 10 years, get 300 back and hey presto you have almost doubled your chapter in time for a nice rebellion.
9. All chapters tend to fight Xenos at some point and their knowledge is kept within the Chapter, specifically sending men to a Xeno fighting Chapter makes no sense if you could have them with you and go do it yourself.
10. If he dies you gain nothing and lose a hell of a lot; a geneseed, a suit of marine armour and whatever promise he had. If he lives you get one marine with some experience of war against possibly a couple of xenos, this is not a good trade.
I short I can only see the DeathWatch in a negative light, if they had been created as a Chapter much like the Grey Knights then I believe the unit could have become a brilliant addition to 40K. As it is I view them as a marketing gimmick designed to give SM players access to multiple chapter insignia and a little extra punch against some foes, but on the whole completely unnecessary.
I hope to turn this into a thread of useful discussion so thanks for contributing