I have the same concern... why can't the UFX be adjusted in the 0-10db range in 1db increments? The first test I did with the UFX was to record some drums.

I used a Shure 57a Beta on snare (industry standard mic for a snare drum).

Result:
0db too quiet - only just going in to yellow at full pelt on the drums.
+10db is too loud - giving occasional clipping.

8db would be perfect, why is there no 8db???

The only reference I could find in the manual (with no explanation, which is ambiguous at best, misleading and doesn't mention the first 10db of the mic pre stage being unusable!!) was this:

"The four balanced microphone inputs of the Fireface UFX offer a digitally controlled gain of 0 to
65 dB via XLR/TRS combo jacks. The gain range is 65 dB, adjustable in 1 dB-steps over a range
of 55 dB. The soft switching, hi-current Phantom power (48 Volt), switchable per channel,
provides a professional handling of condensor mics. Up to a level of +12 dBu, the front XLR
input can also be used as Line input."

So what is the reason? Can it be rectified?

It is because the IC pre-amp (Texas Instruments PGA2500*) used in the front end is designed that way.  Its gain can varied by digital control in 1dB steps between 10 and 65dB.  It also has a separate control to set it to unity gain (0dB).  Thus there is no provision to set a gain value between 0dB and 10 dB.

So if you need further explanation, you should talk to Texas Instruments. And I endorse Daniel's remarks - the performance at either 10dB or 0dB gain is such that recording 10dB low would not have a noticeable impact on noise and distortion.  (And would you run the snare at 0dBfs in the mix anyway?)

(*BTW, the BabyFace uses a cheaper version of the above chip - it has a range of gain adjustment of either 0dB or 9dB to 60dB in 3dB steps.)

There really is no point. If you're concerned about bit resolution, don't be. And I don't know of any mic preamps that has a broader gain range than 55 dB. So usually, if you need less than 15-20 dB's of gain, you must pad the signal and up the gain. What you do with the UFX is "pad" to no gain and up it in software instead. You do not lose any resolution or performance. 24 bits have headroom to spare and beyond...