But with a pretty low limit on daily spend with the contactless card, it effectively has limited value too - assuming you'd cancel it within 24 hours of it being lost or stolen in most cases - and is therefore difficult to 'cash out' on for a thief. So actually both do apply to your contactless debit card, really.
Actually there's likely more incentive to pickpocket a wallet for its cash contents, rather than the contactless card(s), given in a lot of wallets this will be both greater and, by definition of course, easier to cash out on! So it seems like carrying contactless cards doesn't add any particular incentive for theft above what has existed forever with carrying cash. Also, any other security vulnerabilities with the debit card, unrelated to contactless, still exist exactly as they do today.
So, it seems to me that merging cards, i.e. adding contactless and the ability to use it for transport, and other things, seems to just add marginal convenience with essentially no added risk of theft or fraud[1]. So yes, it's not a life-changing technology and it won't mean you no longer have to carry a wallet, but it represents a small win and certainly a step improvement for everyday convenience, so that's good right? I for one simply think the fewer cards I have to carry, the better, to be honest.
[1] though I have, to be fair, heard about the issue of thieves 'skimming' contactless cards in people's pockets, I don't know if that was always just a scare story or if it's actually a real exploitable vulnerability - but this seems likely to be a pretty far-flung edge case anyway.