The musician - who will star as a mentor in ABC's new reality singing competition Duets - says that he thinks that the Fox show will be "interesting" with the former Disney stars on the panel.
Legend told E! News: "You know, it's interesting. It should be very interesting. I don't have much to say. It's interesting when people you know... are judging singing competitions when they aren't really singers. It's interesting."
However, Legend's Duets co-star Robin Thicke has supported both Spears and Lovato.
Thicke said: "Oh come on, everything Britney does, she excels at. Demi is a lovely young lady, and I'm sure they'll do nothing but make the show a lot more fun to watch."
Kelly Clarkson - who will also be working with the pair on the upcoming talent show - agreed that the two bring positivity to the program.
She added: "I'm excited for both of them. I'm a big fan of both of them. Both of those girls are super sweet.
"I think it's fun for them too to just be more out in the public eye again. I think those two are two you really want to root for and you really want them to succeed, so I think that's exciting."
The original American Idol winner continued: "Obviously they are both performers so... I think that show really needs it. I don't think everybody latched onto that show like they thought they would."
Meanwhile the first American X Factor winner Melanie Amaro revealed that she thought the show's boss made the right decision at a showcase for former X Factor UK contestant Cher Lloyd.
Amaro said: "I'm really happy Simon chose two wonderful people. I think Demi's going to bring that youthful thing, and then because Britney's so experienced in the music business, I think she'll do very well. I'm very happy and I'm looking forward to both of them!"
Watch the music video for 'Ordinary People' by John Legend below: