I cloned the repo, checked out to v2.1.3 and ran composer install. It all seemed fine, then at the end I got these notifications:
symfony/intl suggests installing ext-intl (to use the component with locales other than "en")
symfony/form suggests installing symfony/framework-bundle (For templating with PHP.)
symfony/security-core suggests installing symfony/expression-language (For using the expression voter)
symfony/security-core suggests installing ircmaxell/password-compat (For using the BCrypt password encoder in PHP <5.5)
symfony/twig-bridge suggests installing symfony/templating (For using the TwigEngine)
symfony/twig-bridge suggests installing symfony/security (For using the SecurityExtension)
symfony/twig-bridge suggests installing symfony/expression-language (For using the ExpressionExtension)
symfony/validator suggests installing egulias/email-validator (Strict (RFC compliant) email validation)
symfony/validator suggests installing symfony/expression-language (For using the 2.4 Expression validator)
symfony/var-dumper suggests installing ext-symfony_debug ()
m6web/symfony2-coding-standard suggests installing m6web/coke (PHP CodeSniffer configurator)
symfony/dependency-injection suggests installing symfony/proxy-manager-bridge (Generate service proxies to lazy load them)
symfony/http-kernel suggests installing symfony/class-loader ()
symfony/routing suggests installing symfony/expression-language (For using expression matching)
rossriley/flysystem53 suggests installing predis/predis (Allows you to use Predis for caching)
rossriley/flysystem53 suggests installing league/flysystem-eventable-filesystem (Allows you to use EventableFilesystem)
rossriley/flysystem53 suggests installing league/flysystem-rackspace (Allows you to use Rackspace Cloud Files)
rossriley/flysystem53 suggests installing league/flysystem-copy (Allows you to use Copy.com storage)
rossriley/flysystem53 suggests installing league/flysystem-azure (Allows you to use Windows Azure Blob storage)
rossriley/flysystem53 suggests installing league/flysystem-webdav (Allows you to use WebDAV storage)
rossriley/flysystem53 suggests installing league/flysystem-aws-s3-v2 (Use S3 storage with AWS SDK v2)
rossriley/flysystem53 suggests installing league/flysystem-aws-s3-v3 (Use S3 storage with AWS SDK v3)
rossriley/flysystem53 suggests installing league/flysystem-dropbox (Use Dropbox storage)
rossriley/flysystem53 suggests installing league/flysystem-cached-adapter (Flysystem adapter decorator for metadata caching)
rossriley/flysystem53 suggests installing league/flysystem-sftp (Allows SFTP server storage via phpseclib)
rossriley/flysystem53 suggests installing league/flysystem-ziparchive (Allows you to use ZipArchive adapter)
siriusphp/upload suggests installing knplabs/gaufrette (Alternative filesystem abstraction library for upload destinations)
monolog/monolog suggests installing graylog2/gelf-php (Allow sending log messages to a GrayLog2 server)
monolog/monolog suggests installing raven/raven (Allow sending log messages to a Sentry server)
monolog/monolog suggests installing doctrine/couchdb (Allow sending log messages to a CouchDB server)
monolog/monolog suggests installing ruflin/elastica (Allow sending log messages to an Elastic Search server)
monolog/monolog suggests installing videlalvaro/php-amqplib (Allow sending log messages to an AMQP server using php-amqplib)
monolog/monolog suggests installing ext-amqp (Allow sending log messages to an AMQP server (1.0+ required))
monolog/monolog suggests installing ext-mongo (Allow sending log messages to a MongoDB server)
monolog/monolog suggests installing aws/aws-sdk-php (Allow sending log messages to AWS services like DynamoDB)
monolog/monolog suggests installing rollbar/rollbar (Allow sending log messages to Rollbar)
behat/mink suggests installing behat/mink-zombie-driver (fast and JS-enabled headless driver for any app (requires node.js))
guzzle/guzzle suggests installing guzzlehttp/guzzle (Guzzle 5 has moved to a new package name. The package you have installed, Guzzle 3, is deprecated.)
phpunit/php-code-coverage suggests installing ext-xdebug (>=2.0.5)
phpunit/phpunit suggests installing phpunit/php-invoker (~1.1)
codeception/codeception suggests installing codeception/phpbuiltinserver (Extension to start and stop PHP built-in web server for your tests)
Some of theme seem innocuous, but there are a few that seem important:
symfony/templating
symfony/security
and a few others
Are these essential? If so, shouldn't they be in the composer.json file?
In aspect of Bolt's core they are really just suggestions. There are no known harms or degradations without them.