Demo spec is on raids mostly to replace Flametongue Totem/Totem of Wrath's spell power buff so glyph of life tap is must. Some warlocks even play haste/spirit offsets instead the classic haste/crit to boost their spell power and raid in general by even more.
About the pet it's situational and sometimes imp is even better on fights like dbs where you need higher damage below 35% hp (+5% fire spells crit - soul fire, immolate, incinerate,..) and you're able to drop your Intensity talent for 0.5s reduce (2s cast) fireball because unlike felguard Imp's attacks are 1. stronger and 2. cannot be dodged
Imo demonic empowerment is useless if you're playing felguard. 15 seconds buff (compared to 30s, 20% crit for Imp) with 27 off for just 20% attack speed on miserable pet damage is better to be replaced with 1/3 on Supression so you can reach the sweet 14% hit from only gear (amulet of silent eulogy, t10 gloves, plague legs, icc ring and valanar's other signet make exactly 13% hit) without gemming/enchanting it (especially if you're horde without possibility having draenei in your party group)
Imp spec for short, no pushback fights
Felguard spec
About your question the only scenario where glyph of felguard will be better than meta is if you're playing without 3/3 nemesis which is **** to even think about on anything but dbs and similar short dummy fights
it's not that much about the second immolation aura but the off gap you're shortening by additional 6 seconds on every usage
nemesis+glyph: 36 seconds up, 126 seconds cooldown = up again after 90 seconds from expiration
no nemesis/glyph: 30 seconds up, 180 seconds cooldown = up again after 150 seconds from expiration
so 1 minute difference total..do the math on long fights like lod