The Underarm serve in top level Mens' tennis. Legit or disrespectful?

Kyrgios gets booed for it but won't stop using it.
Britain's two most famous living British mens players have voiced contrasting opinions about the tactic, as reported by the Express.

Tim Henman has labelled Andy Murray’s use of the underarm serve as “strange” and “shocking” as the Brit channelled his inner Nick Kyrgios during his Wimbledon first-round win over James Duckworth. Murray has defended his use of the shot but Henman believes the Scot got lucky with his serve.
"I haven’t seen it much from Andy,” Henman said during the BBC’s coverage of Wimbledon.” In my opinion it is such a shocking tactic, especially on a grass court, you’re inviting someone to come forward.
“If your opponent is standing so far behind the baseline then you might feel like it is the opportunity to hit a drop shot. But for me personally, someone who likes to come to the net, I think it is a pretty strange tactic but Andy did seem to get away with it.”

Kyrgios has explained in the past that it creates an opportunity for him to drop in an underarm serve every now and again to prevent his opponents from creeping too far back.
And Murray can't understand why people find it "disrespectful" when it's a viable tactic if the opportunity presents itself. "He changed his return position. That's why I did it," Murray said. "He was standing very close to return. He was struggling a little bit on the first-serve return, so he stepped probably two meters further back. As soon as I saw him step further back, I threw the underarm serve in. I personally have no issue with players using it. I never have. Certainly more and more players have started returning from further, further behind the baseline now to give themselves an advantage to return. The underarm serve is a way of saying, If you're going to step back there, then I'm going to possibly throw that in. I don't know why people have ever found it potentially disrespectful or... I don't know. I've never understood that. It's a legitimate way of serving. I would never use an underarm serve if someone was standing on the baseline because I think it's a stupid idea because they're going to track it down and it's easy to get. "If they stand four or five meters behind the baseline, then why would you not do that to try to bring them forward if they're not comfortable returning there? Tactically it's a smart play. No one says it's disrespectful for someone to return from six meters, whatever, five meters behind the baseline to try to get an advantage.
So I used it not to be disrespectful to him but to say, If you're going to step further back to return the serve to give yourself more time, then I'm going to exploit that."

Social media reacted quickly to Murray's use of the tactic and seemed pretty polarised.

Would be interested in canvassing any thoughts on here. Legit or disrespectful?
Should it be encouraged to be adopted by more players or are the optics bad for the sport?