Nothing against the pop singer or any other prospective guest personally - he just doesn't think the show should bring guests aboard for a ratings boost alone.
"I'm in the minority that our show does not need stunt casting in order to succeed," Harris declared during a break from taping the Monday night CBS sitcom, which has grown a cult following since its 2005 debut.
"I worry that if they start 'Will and Grace'-ing us too much, that the show will suffer. And we're all really proud of the content of the show. I mean, viewership is not our game. It's the network and the studio's game, you know. It's the promotion department's game," the actor, who plays womanizer Barney, told The Associated Press in an interview last week.
Spears has been the most high-profile guest star on "Mother," following appearances by Mandy Moore, Enrique Iglesias and Heidi Klum. Her well-reviewed March 24 cameo as a bubbly receptionist drew 10.6 million viewers, up 1 million from the previous week.
The show has an average viewership this season of 8 million per episode, including reruns; last season, it attracted an average of 8.5 million viewers each showing. It returned with all-new episodes in March following the writers strike, capturing 9.6 million viewers the week before Spears' appearance and slipping back to 9.5 million the week after.
Executive producer Carter Bays told the AP he'd be open to another Spears guest spot. As it happened, the pop star's camp approached the show, and Bays is proud of the result: "We had no illusions about what the stakes were. And if it was classic stunt casting, like, 'Oh, my tour bus broke down outside' and I just go, 'Wow, Britney Spears, what are you doing here?' then, like, we're just flying over that shark real fast. It was a great character and I think she played it well. ... I'm glad that we didn't sensationalize the character in any way."
The show concluded with Spears' secretary flirting with Barney. The following episode, which aired last Monday, introduced a "mystery woman" who's been warning other women not to date the single-minded cad. Asked if Spears might turn out to be the secret slanderer, Bays said: "Could be. There's no reason why not." He also suggested the talented actress Sarah Chalke, who guest starred alongside Spears.
Asked for his guess, Harris said: "No telling, but based on the stunt casting we've done in the past, I'm guessing Tara Reid."
"How I Met Your Mother" stars Harris, Josh Radnor, Cobie Smulders, Alyson Hannigan and Jason Segel as an urban family of twentysomething New Yorkers. A familiar premise (see: "Friends"), yet the show has stood apart through its own brand of clever writing and in-jokes, cast chemistry and signature use of flashbacks.
Still, it lacks the broad appeal of shows like "Friends," which had a celebrity cast and a string of celebrity guest stars, and fellow CBS sitcom "Two and a Half Men." It doesn't have the industry recognition of NBC critical darlings "30 Rock" and "The Office." Its only major Emmy nomination went to scene-stealer Harris last year for outstanding supporting actor in a comedy series.
"The challenge for the show has always been to get more eyeballs on it," said Radnor, who portrays unlucky-in-love architect Ted. "The only thing - and this is not a slam on any of the other shows on CBS on Monday - but the only thing we have in common with those shows is we're half an hour, multi-camera shows with a laugh track."
Radnor compared the show to an under-the-radar, yet-to-be-discovered band.
"That's why the people who love this show looooooove this show because I think it's like your favorite band that hasn't gotten popular enough for you to start hating it," he said. "So you can still kind of love it and wear the T-shirt and speak in code with the other people who are on to it, but it hasn't tipped into this kind of phenomenon where you start to turn on it."