I think there was a HL A310 some years ago - the crew demonstrated that not only can flying with the gear down make it impossible to reach an alternate - it can even make it impossible to reach the destination - despite the FMC predictions
So before you drop the gear - check what your revised Alternate & Final Reserve Fuel is gonna be - this info can be found in the QRP. The same goes for operation of the SBY flaps - make a calculated "guesstimate" of your fuel required before touching those switches.
That being said - depending on when you discover the failure & the amount of fuel you have chose to carry, you might find yourself way below those figures when you finally realise you have a problem. E.g.: 1 main gear will not extend when you call for gear down. Will you retract the gear on the go-around? If you do - that might compound the problem. If not - you might not have fuel to go anywhere else. Do you take that into consideration before you "go & have a look" at an approach that you are not totally convinced will lead to a landing?
As OS pointed out - when you know the problem in advance, you have options, and need to exercise them. The problem occurs when the problem is sprung on you - so you even need to take these considerations into account during normal ops. Trying an ILS cat I with RVR0600 BKN001 - while legal - suddenly becomes less appealing if you are close to your minimum fuel for diversion already, and you think about suddenly getting a flap skew around F25
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