Thing is, its really not just 1 turn. Also, if you have a lance of mechs with them in a company, the advantage of the smoke becomes
much more powerful. Frankly, in the last 5 years, I have seen an Ultra jam TWICE in my group. I would, generally, not fear that
roll of a 2. Also, if you are using HVACs, you are likely using a custom unit, anyway..and so, CASE II would, thus, be available.
The problems with this are:
A) There are better ways to generate smoke that don't involve you accidentally blowing up once and a while. You can get a regular AC and a Flamethrower for less weight than a comparable HVAC. And then you don't accidentally blow up once and a while. And still generate less heat.
B) The more HVACs you have in a lance, the more likely one is to blow up. If you have 4 guys each with an HVAC, each firing every turn, on average, one of them will blow up in 9 turns. Which is a totally reasonable possibility in an average game. So by volunteering to use the HVACs, you are just giving your opponent essentially a free kill (well, mission kill, anyway, even with CASE--HVACs are heavy, so probably the main gun on a given unit).
Is it nice that HVACs generate smoke? Yeah, I guess--if there was a gun that generated smoke without accidentally blowing up while also shooting someone, that would be worth using occasionally. But as you can generate smoke in other ways while also shooting someone with other, non self-destructive guns, probably not the best plan to follow just to be able to make smoke while shooting.