In other words, if it cannot be discussed using concrete language (e.g. God), then it is better left unspoken of. Considering all the things that we cannot speak of definitively, but that are very important to the human experience (morality, love), is this good advice? If so, how do we contend with those things we 'cannot speak of'?
There is a clear tension in the Tractatus between its main doctrine - we may call it proto-Logical Empiricist - and the final mystical assertions.
6.53 The correct method in philosophy would really be the following: to say nothing except what can be said, i.e. propositions of natural science — i.e. something that has nothing to do with philosophy—and then, whenever someone else wanted to say something metaphysical, to demonstrate to him that he had failed to give a meaning to certain signs in his propositions.
6.421 It is clear that ethics cannot be put into words. Ethics is transcendental.
With ethics (and the "problems of life"), also the "mystical" is surfacing; see 6.44.
4.12 In order to be able to represent logical form, we should have to be able to station ourselves with propositions somewhere outside logic, that is to say outside the world.
6.54 My propositions are elucidatory in this way: he who understands me finally recognizes them as senseless, when he has climbed out through them, on them, over them. [...] He must transcend these propositions, and then he will see the world aright.
Firstly, I think what Wittgenstein meant by the expression and what he later thought of it might not be one and the same, and so looking further into his work might not yield an honest interpretation. This was virtually the last thing he wrote before effectively retiring from philosophy for nearly a decade, and the words were partly responsible for initiating the Vienna Circle of Logical Positivists, so I think it is is a reasonable presumption that he did quite literally mean that we should not discuss things which cannot be expressed in concrete language, rendering most of philosophy and virtually all of metaphysics pointless. That's almost certainly what philosophers like Carnap though he meant and Wittgenstein did nothing at the time to disabuse them of that notion.
The question of whether that is good advice or not depends on what you include in the list of things about which we cannot speak. Such a conclusion is usually reached by the listener, not the speaker. If someone talks about God in a philosophical context, for example, they have usually decided that it is something about which they can use concrete language. Maybe they are a committed Rationalist and so believe that the statements they are making from their rational analysis are indeed concrete. Likewise, a committed physicalist might make what they believe to be concrete statements about love on the evidence of neuroscience.
It is probably good advice, but it's like asking people if they are a good driver or have a good sense of humour, most think they are above average (which is of course impossible). Most people will think that the thing they are about to say is concrete and meaningful, but the listener might disagree and the matter cannot be resolved.
What will ultimately determine the question of what one can speak, will be the utility of the statement. If it contains conclusions based on concrete things, then it will most likely be useful (in that the predictions made by it will be accurate), if it is vacuous then it will be less so (as it's predictions may only be accurate by chance).
Given the direction which Wittgenstein turned thereafter, it might be good idea to simply first take the statement literally, and not ascribe interpretation (e.g. about reference to God) to it. Taken that way, it is simply a logical necessity, so it can be neither a practice, nor advice, good nor bad.
So then, how does it apply in context? If we look forward from the Tractatus to where he is going next, the period between the Tractatus and the Investigations where his lectures were developing the notion of the language-game, then logical discourse itself, as laid out in the Tractatus, is simply an example of a single clear game, if one from which numerous others inherit.
In that context, this tautology comes to reflect the respect for the boundaries between language games. Within a given game, there are things to which the rules simply don't apply. One can make those moves, but they are either pointless, or they are disguised trickery, and either way, they are not useful participation in the ongoing dialog.
In this way, I think Wittgenstein returned to his original position, when he claimed the Tractatus would be dedicated "To the glory of God", if that would not be instantly misunderstood. In that frame of mind, there is a reasonable place to discuss God, as long as one realizes the purpose of doing so, and does not use the power of that domain to corrupt other domains of discourse.
Looking forward to the theory of language games, this is fitting, because it acknowledges that purposely crossing clarifying boundaries and bringing over power acquired in a different domain, is a kind of warfare against the topic in the name of contribution.
It seems to me that you are making an uncalled for switch between things about which one cannot speak and things about which one cannot speak definitively, and these are distinct "things".
That is to say, not being able to define morality or love does exclude you from speaking about them.
On the other hand there is a "thing" about which one cannot speak. In fact, it appears that most people don't know what the hell the people who do speak of that "thing" (about which one cannot speak) speak about.
It is very difficult to verbalize it. To say something about meditation is a contradiction in terms. It is something which you can have, which you can be, but by its very nature you cannot say what it is.