So its not taboo and its what works for you BUT, i think it just adds unnecessary complexity - both in a sound (balancing gain - valves add colour even when clean) and logisitics ( more wires, stuff to lug around).
Many people do this to provide what they feel is "missing".
1) My focus has changed to FOH because I don't mike up now...so I don;t need to get a great sound on stage to translate through the PA
2) I'll take 95% of my sound and less hassle then add this overhead. I love that my sound is consistent ( SS power is more linear than valves-which is part of their charm but farting around finding the sweet spot for me is now just irritating) and reliable, adding valves sort of defeats the point and the KPA becomes more of an expensive FX unit. I like the one stop shop in a box, hence I also don;t use any external pedals.
But that's just me....