don't need any statistics to back this up, just listen to it. like I said, the worst problem with the built in sound (apart from the digital i/o which is fantastic) is the very obvious interference you can clearly hear from the workings of the machine bleeding into the audio signal. apart from that, the converters are not great, which is also pretty easy to hear - poor stereo spread, brittle tops with lack of definition and murky bottom end. all you need to do is A/B it some time and you'll hear the difference, even in average speakers. try listening to the spdif output through any D/A converter built for audio work and compare it to the analog output. world of difference.
this isn't to say _at all_ that you can't work with the built in analog sound - you totally can, it just depends on what you are working on at the time. for writing/composing/developing your music it's more than fine. when you are working on your music, your focus is on the writing, arranging and development. plus, as someone else correctly stated in this thread, internal synths and bouncing are totally unaffected by the quality of the output you are hearing because it is all internal. so for this part of the process, it's workable and fine.
what
is very affected though, is
your ability to hear accurately what you are working on in terms of the precise tonal characteristics of your sounds. like I said, for writing, it doesn't really matter because you are engaging with your material on a different level at that stage. but when it comes to mixing and mastering, it becomes very important. if you aren't hearing the tonal qualities of your sounds properly when mixing, you will make all kinds of mistakes and overcompensations that will make it harder to get a good mix. it's the same argument that explains why studios use the best quality, faithful sounding equipment at every stage of the signal chain when it comes to monitoring. people often make the mistake of downplaying the importance of this because things will end up being played in ordinary systems, like a car or club or on radio. but it's all the more important for that - when mixing, you need to be hearing what is really going on in terms of dynamics, EQ, pan etc.. so that your treatment is creatively applied to get the sound you want, and so that you aren't inadvertently adding buckets of EQ to sounds in the wrong way because your monitoring is not letting you hear what is really going on. for example, if you've not been able to accurately here and process your kick drums and bass sounds because your monitoring is just all wool and mud down low, the problems will show up even more out in the real world of TV speakers, clubs and car stereos..
as you said, when someone asks if the internal card is good enough "for now" the real question is, what are you trying to use it for? if you are just writing/composing/getting your music together, then yes it probably is. but if you are wanting to record external sources with any decent amount of confidence that what you capture is going to be usable and clean, then it may not be.
and if you are intending to get stuck into mixing your stuff to a completed product stage, then you need to take monitoring seriously, a big part of that is the quality of the i/o from your DAW.