Is case grounding compulsory in typical 32.768kHz crystal for Real Time Clock?
One reason may be more mechanical than electrical. Crystals are mechanically vulnerable, and may easily be damaged by shock and (ironically) vibration. Fixing the housing to a larger mass may reduce the effects of those.

Note that, while most likely there will be electrical contact between case and ground in the second picture, I would not rely on it without adding a solder blob at the point of contact. Check it out: hold your DMM's probes on the case to measure resistance. You may have to slightly rub over the surface to see the expected 0 &Omega;.