Why a rogue 'splash' is better than a 'pure' rogue:
If someone has ONE lvl of rogue, its virtually guaranteed they are a trapper. If they have 2 lvls, they may be a trapper, or they may have it just for evasion. More likely for both.
If someone has multiple lvls of rogue, there is a high chance they are running some sort of assassin build, suck at traps, and WONT TELL YOU until you are in the quest and depending on them to do it.
A pure rogue is pot luck; a rogue splash is almost always going to be a trapper. Odds are certainly stacked in that direction.
A rogue splash can do other things. Most common of course is the wiz/rog. If you really need it explained WHY the wiz/rog is such a good combo/valuable asset to the party -- well sorry but I just cant be the one to explain why clouds make rain to you
Why a pure rogue is better than a rogue splash:
A rogue splash can get skills high enough to do their job, assuming they build and gear for it. A pure rogue can get the same skill lvls though enhancements and built-in feats, with less effort. They can get higher skills with the same effort. This means they can do the same job at a lower lvl and/or with less effort than the splash. This is useful if you are trying to complete a quest at a higher lvl or diff than the party level. At level or above? Doesnt matter. If you need a 40 DD for guaranteed success, then it doesnt matter if you have a 40 or a 60. Its just easier/faster for a pure rogue to get to that 40 (assuming they build for it of course).
A pure rogue can also bring the hurt in terms of sustainable DPS (defining this as not requiring use of resources such as cooldowns and SP -- splashes can do this as well, usually using up a resource in the process, so not 'sustainable' per se).
I run wiz/rog -- alot. Love em. Keep tweaking my builds and gear to get an extra bit out of them every chance I get. TRed my wiz/rog into another wiz/rog, changing a few things to improve him. The fact that whenever I join a PUG that is looking for a rog, and the star asks me if I can handle the traps makes me cringe at the fact that there are so many crappy rogues out there that they even feel the need to ask. I die a little inside every time I am slot 6 and someone says 'dude we need a rogue for this quest'. ~sigh~
I also run bard/rog, rgr/rog, and even a barb/rog. Personally I think I shouldnt have to de-rage to disable traps. I should be able to bash the control box with my face (like a non-rog barb would do) and roll my DD check that way.
And I consistently get any at-lvl traps without failure. And I do my 'other job' as well. Now, if a <cough> 'real rogue' joins the group, either before or after me, I let them handle the traps (if they can). I have rogue skills for convenience, not epeen, and for the evasion. If they want trap parts, they can have em. But if you are a sub-par rogue, I will do what is best for the GROUP and get the traps myself. And if you complain about 'stealing your parts' I will tell you exactly what you can do with yourself. First, they arent YOUR parts unless you do your job and get the traps. If you didnt burn a thieves tool, they arent 'yours'. Second, 'stealing parts' implies that I got parts. I didnt. Success of the mission outweighs your desire to risk blowing a trap because you SUCK, on the off-chance you MIGHT disable the trap and get parts from it. And who said they were 'yours' to begin with anyway? Personally I think they belong to the healer O.o