In most shuls I've ever davened in, many people recite the last bracha before Shema at Shacharit and Ma'ariv along with the Shliach Tzibbur, and in some places (notably Chabad), the Sha"tz says those brachot silently. Either way, it's rare to hear someone answering 'amen' to those brachot.
I believe that these practices are based on a misunderstanding of a short Gemara in Brachot 45b. Surrounding that sugya is a dispute, basically between Sephardi and Ashkenazi poskim, about saying 'amen' to one's OWN bracha (see Rashi and Tos ad loc s.v. 'ha', Rambam Brachot 1:16). Sephardim, based on that Gemara, answer 'amen' to their own brachot for every bracha which is at the end of a series of brachot. For example - 'shomer ammo yisra'el la'ad' at ma'ariv is the final of the 4 brachot of shema. yishtabach is the end of the series which began with 'baruch she-amar'. the end of hallel - same thing. End of shemoneh esrei - same thing. Ashkenazim, starting w/ Rabbeinu Tam, hold that the only bracha to which this applies is 'boneh yerushalayim' - the (originally) final bracha of bentching. Since that's where bentching essentially ends, and the fourth bracha was added later, Chaza"l saw fit for one to answer 'amen' to one's owbn bracha, in order to distinguish the earlier, biblical part of bentching from the later, Rabbinic portion. These rules work perfectly with the guidelines that govern 'bracha ha-semucha le-chaverta' as well. ve-acamo"l. (See Shulchan Arukh OC 215:1, 188:1,2 and 66:7)
When it comes to the bracha before Shema - i.e., the 2nd of the 'Birchot Kri'at shema', the Rishonim address why one wouldn't answer 'amen' to their own brachot - namely, because it's not the end of the series, rather, the Shema itself was integrated into the series which only ends after the 3rd (shacharit) or fourth (ma'ariv) bracha. Thus, the Shema itself doesn't constitute an interruption of the series, and one need not say 'amen' to his own bracha.
It seems that this was misunderstood to mean that one should not answer 'amen' to that bracha at all. That doesn't make too much sense, as it's clear from the Mishna down to the Shulchan Arukh (OC 66:5) that an interruption between the first and second brachot and between the second bracha and the Shema itself are equivalent, and everyone says 'amen' after the first bracha, as is encouraged. I haven't been able to uncover where this misunderstanding originated (and Neklaf is no longer posting).
In the meantime, I make sure to answer 'amen' to the 2nd bracha of Shema conspicuously, because it's proper, and because, hey, being a contrarian Rabbi is what I'm all about...