I think it's fine the Beckham's are going, think they're fairly worthy guests too & not in terms of being big celebs or whatever. Remember that aside from all the media obsession with them & the glitz etc he is one of England's best footballers, most caps for an outfield player for England, Captain for years, showed many admirable qualities in his career & won many things at various clubs. She's a success in her own right, maybe their career achievements aren't meaningful to some but to many their accomplishments are inspiring & worthwhile (particularly David's, you rarely hear anyone say anything bad about the way he goes about his football, everything else maybe they moan about but he's always been seen as a true pro in his football, he's an OBE already & many think he'll get a knighthood one day & he's a legend of the modern game). Then factor in all the charity work they've both been involved in, they have their own kids foundation, he's worked with UNICEF for years, bith have supported Comic & Sport Relief over the years & generally I think have done numerous things for charity, now whether this is done just to look good etc is beside the point if you ask me, they're still doing something to try & help people. And then like others have mentioned there is at least some level of personal connection/association between William & David & so to me at least the Beckhams aren't an unexpected invite.
I do hope they don't overshadow proceedings (I really doubt they actually will) of course as the media do love to focus on these two regardless of the occasion or whether they should be the focus etc. Anyway just my opinion, I just don't the fuss over them being invited, I don't think it "cheapens" the event or anything as I do honestly think David at the very least is fairly deserving of being invited & like someone posted, you can't invite one without inviting the husband/wife/partner.