I generally agree with Hobo. There is no such thing as a party that is too weak. There are only parties with are not capable of handling the challenges that the storyteller sets for them (or that they set for themselves).
As for the hypothetical 'all rogue' party, the 3rd edition Rogue is quite powerful and an 'all-rogue' party only plays to its strengths:
a) Everyone in the party wants to flank the target and has the abilities to make this possible.
b) Everyone in the party wants to ambush the target, and this tactic is normally limited by the parties least stealthy member.
I have nothing against an all rogue party. Though, prior to my current campaign I warned the players that rogues might have trouble in long sections of the planned campaign because of the planned importance of monsters immune to critical hits. That being said, had I ended up with an all rogue party anyway, this would have told me something about the parties planned intention to opt out of the story line and I probably would have planned accordingly. Overall campaign events would have still happened, but the party would observe them from a different position in the game world.
The DM that taught me explained he once had to deal with an all thief party in the adventure B2: Keep on the Borderlands that made their primary goal robbing the bank in the Keep. Sometimes you have to adapt.
Sometimes an 'imbalanced' party proves to be really powerful. An all cleric party can be extremely powerful. I got myself into a situation with an all ranged specialist party that tended to mow down any tactical challenge I presented them with without really moving. They were the tactical equivalent of a machine gun nest, and it annoyed the heck out of me (but I also can't say that it wasn't a reasonable solution to the problems I typically through at the party). An all arcane party can be really powerful, either by using spells to move into all the needed roles - summoners can be tanks or healers, shapechangers can be brutes, and of course artillery and battle field control are covered - or by simply overwhelming foes with the ability to go nova all at once or brutal gauntlets of save or suck effects. At low levels, the all fighter party trounces just about everything.
So long as the party is internally balanced, I think you are ok. The real problem you should watch out for is, as others have noticed, a lack of balance between characters. A lack of correspondance between party abilities and some hypothetical average challenge isn't a big problem, at least until the players themselves begin to feel their choices are 'weak'.
My suspicion here is that what is going on is really a failure of the presumed social contract. That is, you seem to be suggesting that your party normally trusts the DM to provide challenges and has the assumption that when a challenge is provided by the DM they must make a reasonable effort to overcome it. Therefore, there is an implicit social contract that the challenges provided by the DM are fair and reasonable. Conversely, you seem to have the stance that player's part of this contract is that they will create characters which collectively have certain abilities that allow you to present them with a broad range of 'standard' challenges. In D&D, for example, it's often assumed by adventure writers that at least one arcane and one divine spellcaster with access to a standard set of utility spells is present in the party. And its generally assumed that they have access to sufficient magical healing that a night's rest can cure most of the days wounds so that a certain high pace of combat and intence challenges can be maintained over the long run. Since your party lacks these abilities, you are feeling that they aren't living up to their side of the bargain.
All of that is normally fine, but I think you should consider that this may be signalling on the part of the players that they would like to change the social contract. For example, the party may be wanting to move out of a passive stance and become more active in setting more of their own story goals. Or they may be signalling that they've become bored with usual sort of challenges. Or they may be signalling a desire to temporarily change the style of play - from gritty to heroic, or from high to low fantasy, or from heroic to anti-heroic, or from high drama to low drama, or from hack-n-slash to melodrama, from exploration of space to exploration of character, ect. Without knowing the details, I can't be sure what they are signalling (if anything).