In fact, as demonstrated fairly recently (e.g. here), the Web is effectively the 'engine' producing the goods for some official applications, with a simple 'web wrapper' interface that requires zero coding, effort or testing for the organisation in question.
Now, the purist in me says that an application should be natively coded and fully tied in with the platform it's running on - such a creation normally ends up smoother and slicker, after all. But I do recognise that for the organisation creating the application for (in this case, the third mobile platform by popularity) Windows Phone, if there's already a fully working, fully secure, fully tested mobile-friendly web site then why go to the huge expense of commissioning, testing and supporting a native application?
Long time readers may remember 'There's a bookmark for that!', with me pastiching Apple's famous catchphrase in highlighting some of the great sites and services available to anyone with a web browser on their phone. Device speeds and browser capabilities have improved a lot in recent years, meaning that mobile-friendly web sites (often the one served up by default when the server detects that you're on a smartphone) can be just as pleasant and functional as the full desktop-aimed versions.
And, with Internet Explorer on Windows Phone calling a bookmark a 'Favourite', it seems that a slightly renamed update of the original feature/concept was in order. The selection of examples below is more to get you thinking - it's not even remotely definitive. But it does show what can be done through the browser on a smartphone these days, especially when a native application isn't available, for whatever reason. The lack of the latter is certainly not a showstopper.
Of course, there's an official Twitter app for Windows Phone, so what am I thinking here? Think of Dabr, effectively a web transcoder over the full Twitter experience, as an alternative to the main app. It certainly gives a lot more options in terms of appearance and font, plus performance can be better, depending on what you're doing, since you don't have to worry about a local app caching tweets and updating its databases.
URL to favourite: dabr.co.uk
Of course, there's an official Twitter mobile site too, but I wouldn't recommend using this as it's white-only (more power hungry) and, most of all, the font used is quite small - lots of detail is packed in but it's hard on the eyes.
Something that's very 2014: a Bitcoin client, a way of checking your funds and sending and receiving the cryptocurrency. No native Bitcoin or Blockchain client, so again it's off to the web we go, to great effect, with the server detecting a mobile client and serving up suitable pages:
URL to favourite: blockchain.info
Covered a week or two ago, the Pinterest client on Windows Phone IS the mobile web site, with just the Internet Explorer toolbar slimmed down. So why not just go straight to the web site directly, if you're already in MSIE?
URL to favourite: pinterest.com
National Rail Enquiries
OK, this is a UK thing, but I'm sure similar national rail systems/sites exist in your country?
URL to favourite: m.nationalrail.co.uk
Actually tapping through to buy a ticket is trickier, since the full desktop site is used, so plenty of zooming in and out is needed - but it can be done!
With Google refusing to code any of its main services for Windows Phone (seemingly on principle), the mobile site is the only way in. Happily, the mobile site is pretty good, and only arguably marginally slower than a dedicated client (depending on connection speed).
URL to favourite: plus.google.com
Having this doesn't fully make up for the lack of a proper Google+ client - but it's close at times!
Online banking (Lloyds)
Taking the example of my own bank, Lloyds in the UK, it's a great case of using all the security and maturity of the web rather than coding for mobile natively and risking a vulnerability creeping in.
URL to favourite: lloydsbank.co.uk
You'll have your own banking site and requirements, but I'd be surprised if there's not a fully functional version that works in MSIE on your Windows Phone.
There are, in fact, several very good third party YouTube clients for Windows Phone, in the absence of an official Google version, but there's still a lot to recommend the official mobile web version - it's very fast, is guaranteed never to lose compatibility and can be accessed on any smartphone.
URL to favourite: m.youtube.com
One of the most recent services to get an official 'application' which is actually just the mobile web site, again I point to the URL and experience here - what more do you need? You might need to vary the URL for your country, of course!
URL to favourite: m.pizzahut.co.uk
Comments welcome if you can think of other great examples. Yes, natively coded applications are often better, but with 'app gap' shouts continuing to ring round the ecosystem, let's not forget the great tool that is Internet Explorer - and the mobile sites lovingly maintained by the various online services.